This fictive book is carefully woven into the plot of the Hitchhiker novels, radio and TV shows and stage plays. Or more precisely, it is the basis of the plot, the common ground, with which DNA interconnects all the muddled events which happen seemingly, but not quite, at random.
The one protagonist who shares the confusion with the reader and is simultaneously the 'hero' of the complete story is Arthur Dent, a common English everyman. His primary objective and in fact the only challenge he accepts is the search for a drinkable cup of tea.
The hitch-hike starts off with the destruction of his house to make way for a bypass and shortly thereafter the destruction of the Earth for precisely the same reason by the first alien race introduced in the story: the Vogons. DNA has knitted together an ingenious plot which implicitly introduces a narrator to the story in the form of the electronic book and a character who knows a lot more about the universe than Arthur or the reader, a field researcher working for The Guide. The name this roving researcher eventually gave himself is Ford Prefect, because as he arrived at Earth "he had skimped a bit on his preparatory research. The information he had gathered had led him to choose the name "Ford Prefect" as being nicely inconspicuous." (Hitch Hiker, chapter 1, p.13)
This joke can only be understood if one knows that at the time DNA was writing Hitchhiker a popular car in England was the Ford Prefect and that Ford simply had considered cars the predominant life form on earth.
The only survivors of the destruction of the Earth are these two persons, as Ford as a typical hitch-hiker finds a way to get aboard one of the Vogon spaceships.
The confusing aspect which causes difficulties in summing up the plot in brief is that DNA often introduces new cultures or even complete galaxies only to illustrate the merest incidence or technological device. For example, the Dentrassis, who are the cooks on Vogon spaceships and said to be "the best cooks and the best drink mixers and they don't give a wet slap about anything else" (Hitch Hiker, chapter 5, p.41) or the Oglaroonians, who are a typical example of people who ignore the size of the universe for the sake of a quiet life. (cf. Radio Scripts, Fit VIII, p.163) (*)
Yet one being might still be important: The Babel fish, which helps getting around a basic problem which most SF stories seem to ignore, namely communication. The Babel fish "feeds on brainwave energy, absorbing all unconscious frequencies and then excreting telepathically a matrix formed from the conscious frequencies and nerve signals picked up from the speech centres of the brain; the practical upshot of which is that if you stick one in your ear you can instantly understand anything said to you in any form of language - the speech you hear decodes the brainwave matrix." (Radio Scripts, Fit I, p.29)
Shortly after the 'rescue' Ford and Arthur are thrown out of the spaceship, because Vogons do not like hitch-hikers. This is the point where two other main characters and the most important scientific gadget are introduced to the story: Zaphod Beeblebrox, Trillian and the Infinite Improbability Drive. Zaphod is Ford's 'semi-cousin' and the President of the Galaxy, which is not really important as the "President in particular is very much a figurehead - he wields no power whatsoever." (Hitch Hiker, chapter 4, p.33)
Zaphod is characterized as very 'cool' and a notorious playboy, but in all decisive moments he behaves rather cowardly. He is egocentric and egoistic and generally very concerned about his ego, but finally he is still a sympathetic character. He is humanoid but had chirurgically added an extra head and an extra arm.
Trillian - or Tricia McMillan - is a woman born on Earth who was picked up by Zaphod at a party in Islington where she previously had met Arthur. These 'coincidences' are reasoned out by the scientific invention, the Infinite Improbability Drive which "is a wonderful new method of crossing vast interstellar distances in a mere nothingth of a second, without all that tedious mucking about in hyperspace. (...) The principle of generating small amounts of finite improbability (...) were of course well understood (...)" (Hitch Hiker, chapter 10, p.68-69)
The generation of an infinite improbability generator was made possible by calculating exactly how improbable such a generator was and creating this finite improbability.
This invention makes it possible to move along a higher axis. DNA alludes to the fact that mathematically infinitely more than three dimensions can be calculated and as conventional propulsion methods accelerate in the first three dimensions, The Infinite Improbability Drive accelerates along which DNA calls the Improbability axis.
Zaphod had stolen the The Heart of Gold, a ship prototype equipped with this 'engine' when he was meant to launch it and DNA establishes the characters on this ship together with an android Marvin, who is another prototype. A prototype of a "new generation of Sirius Cybernetics Corporation robots and computers, with the new GPP feature" which stands for Genuine People Personalities, i.e. human behaviour in chatting, expressing feelings and getting on nerves. (cf. Hitch Hiker, chapter 11, p.74)
Marvin is the first experiment of this personality programming and he has an exceptionally powerful brain and an almost infinite capacity for mental activity of all kinds except for happiness. Therefore he is manically depressed as the depressive factors are infinitely higher. He is often nicknamed the Paranoid Android.
The goal Zaphod pursued with this theft remains unrevealed very long, there are only some slight indications that there is a higher plot which determines the actions and events, e.g. on Magrathea Zaphod mentions modifications in his brains someone had made in order to make him forget why he wanted to become President.
The first destination is the planet Magrathea, the home of a specialist industry created "far back in the mists of ancient time, in the great and glorious days of the former Galactic Empire": "custom-made luxury planet building." (Hitch Hiker, chapter 15, p.89)
In orbit around Magrathea the ship is attacked by two nuclear missiles which turn into a bowl of petunias and a sperm whale as Arthur turns on the Infinite Improbability Drive as a last escape from certain death shortly before the impact.
On Magrathea they meet Slartibartfast, a man who designes coastlines and has a compassion for fjords. This man will become important later in the plot when Arthur and his companions save the universe. From him we learn that the Earth was built by the Magratheans for a super-intelligent race as a gigantic computer in which life itself is part of the calculation matrix. The sole task the Earth had to solve is to find The Question. Those pan-dimensional beings had built a computer, Deep Thought, which calculated the answer to life, the universe and everything which was in fact 42. Deep Thought explained that one could only understand the answer if one knew the appropriate question and as it was impossible for Deep Thought to calculate it, the Earth had to be built. But the ten-million program of the Earth was unfortunately interrupted five minutes before its completion by its destruction. Hence the question remains undiscovered.
The following events and their order varies in the different versions of the Hitchhiker epic and I will concentrate on the essential points. Having left Magrathea and heading towards The Restaurant at the End of the Universe to have lunch, The Heart of Gold is attacked by the Vogons. Due to a technical problem, i.e. the computer is jammed with Arthur's wish of a drinkable cup of tea, they need help from Zaphod's Great Grandfather - a ghost. From him we get to know that Zaphod only became President to steal The Heart of Gold and with its help to find the one who really rules the universe.
Zaphod Beeblebrox is caught by Frogstar fighters, a kind of galactic police, and is, as a sentence for stealing The Heart of Gold, brought to The Total Perspective Vortex which "can annihilate a man's soul! The treatment lasts seconds, but the effect lasts the rest of your life!" (Restaurant, chapter 8, p.52)
It "derives its picture of the whole Universe on the principle of extrapolated matter analyses.
"To explain - since every piece of matter in the Universe is in some way affected by every other piece of matter in the Universe, it is in theory possible to extrapolate the whole of creation - every sun, every planet, their orbits, their composition and their economic and social history from, say, one small piece of fairy cake." (Restaurant, chapter 11, p.63)
So when it is turned on one sees in one instant the whole infinity of creation and oneself in relation to it and this shock completely annihilates the brain. (cf. Restaurant, chapter 11, p.64)
But luckily Zaphod is not in the real universe, but in an artificial fascimile and therefore leaves The Total Perspective Vortex unharmed. This artifical universe is the property of Zarniwoop, a man who originally planned with Zaphod Beeblebrox to visit the man who rules the universe and we learn that the whole thing was staged for one single purpose: To bring The Heart of Gold to Zarniwoop as no other ship is capable of getting to this man.
Eventually Zaphod, Trillian, Arthur and Ford find themselves in The Restaurant at the End of the Universe. How they get there is not clearly explained and varies largely in different versions. Anyway, this establishment "is one of the most extraordinary ventures in the entire history of catering.
"It is built on the fragmented remains of an eventually ruined planet which is (...) enclosed in a vast time bubble and projected forward in time to the precise moment of the End of the Universe.
"(...)In it, guests take (...) their places at table and eat (...) sumptuous meals whilst watching (...) the whole of creation explode around them." (Restaurant, chapter 15, p.80)
But here again we miss the great moment of final dissolvement as the characters rush to the car park where Marvin awaits them and they steal a ship, which brings them towards another catastrophe, in which this ship gets destroyed. They escape, but are now separated. Ford and Arthur get aboard a ship that 'evacuates' "an entire useless third of the(ir) population" (Restaurant, chapter 25, p.147), or "a load of useless bloody loonies" (Radio Scripts, Fit VI, p.120) as Ford calls them, from the planet of Golgafrincham and which was programmed to crash-land on prehistoric Earth.
The visit of the man who rules the universe does not give much insight into how or why the universe works, we merely find out that the one who rules it lives in a shack and shows no interest in the universe whatsoever. According to DNA's definition how government should function he is presumably the best one to find for governing the universe.
"The major problem - one of the major problems, for there are several - one of the many major problems with governing people is that of whom you get to do it; or rather of who manages to get people to let them do it to them.
"To summarize: it is a well known fact, that those people who most want to rule people are, ipso facto, those least suited to do it. To summarize the summary: anyone who is capable of getting themselves made President should on no account be allowed to do the job. To summarize the summary of the summary: people are a problem." (Restaurant, chapter 28, p.152)
These are the main aspects of the plot as to be found in the radio and TV series. DNA now proceeds with more tightly knitted and more conclusive events. The third novel begins with Arthur having been alone on prehistoric Earth for five years. Then Ford shows up and they escape through a space-time instability to the "middle of the pitch at Lord's Cricket Ground, St John's Wood, London, towards the end of the last Test Match of the Australian Series in the year 198-, with England needing only twenty-eight runs to win." (Life, chapter 2, p.18)
At this point they meet again with Slartibartfast who is now doing some part-time work for the Campaign for Real Time, which tries to stop problems which arise with the increasing popularity of time travel. Slartibartfast owns the Starship Bistromath, equipped with The Bistromathic Drive, yet another revolutionary new way of propulsion which is based on the observation that numbers go along the theory of relativity of Einstein and especially well in small Italian bistros. The new understanding of the behaviour of numbers, called Bistromathics, made this invention possible. (cf. Life, chapter 7, p.39)
Since they met Slartibartfast they try to inhibit a group of robots from gathering together a key which can release the planet of Krikkit from its envelope of Slo-Time, inside which life continues almost infinitely slowly. All light is deflected round the envelope so that it remains invisible and impenetrable. Escape from the envelope is impossible unless it is unlocked from the outside.
The planet of Krikkit was sentenced to this perpetual exile as the inhabitants, under the influence of the extremely powerful and aggressive computer Hactar, developed into a race which profoundly believed in "peace, justice, morality, culture, sport, family life, and the obliteration of all other life forms." (cf. Life, chapter 14, p.76)
Hactar was a gigantic spaceborne computer. It was the first to be built like a natural brain, in that every cellular particle of it carried the pattern of the whole within it, which enabled it to think more flexibly and imaginatively, and also, as it was pulverized by its creators, because he objected to building a working Ultimate Weapon which would destroy the whole universe at once, not to get destroyed but to keep his consciousness even as a dust cloud wrapped around the planet of Krikkit.
In the course of their efforts to find parts of the key to the Slo-Time envelope before the Krikkit robots, Arthur is confronted with an interesting creature of the Hitchhiker saga, Agrajag, who eventually brings him into a huge cave he craved out of a mountain only for this single purpose. Agrajag is a soul that over millions of years and in the whole universe reincarnates and goes through one decisive thing in any of his many lives: he gets killed by Arthur Dent. Agrajag was many flies he killed, some of them were even swatted with the skin of a rabbit which had been Agrajag again. Ants or newts Arthur stepped on, a bowl of petunias which was called into existence several miles above the surface of Magrathea, all these beings were in fact one person who at this point of the plot threatens to kill Arthur with the help of his "last body. [His] last life. (...) [His] revenge body. [His] kill-Arthur-Dent body. [His] last chance." (Life, chapter 18, p.94)
As Agrajag points out the many of his pointless deaths, he casually mentions an incident on Stavromula Beta where someone should have tried to assassinate Arthur and he ducked and the bullet hit him instead. But as Arthur has not yet been there, it is logically impossible for Agrajag to take revenge on Arthur now and in fact for anything to kill Arthur before he is again directly or indirectly responsible for Agrajag's death there. (cf. Life, chapter 18, p.96)
So the inevitable happens and Agrajag's last body gets killed when it tries to blow up Arthur together with the complete mountain in which they are. But Arthur escapes and learns to fly. DNA lets the Guide explain how anyone can learn to fly as follows: "There is an art (...) or rather a knack to flying. The knack lies in learning how to throw yourself at the ground and miss.(...) All it requires is simply the ability to throw yourself forward with all your weight, and the willingness not to mind that it's going to hurt. (...) One problem is that you have to miss the ground accidentally. (...) You have to have your attention suddenly distracted by something else when you're halfway there, so that you are no longer thinking about falling, or about the ground, or about how much it's going to hurt if you fail to miss it." (Life, chapter 11, p. 58)
These two events, Agrajag assuring Arthur a lasting life until he was on Stavromula Beta, and Arthur's new ability to fly, become very important in the further development of the plot, i.e. in the last two novels of the Hitchhiker 'trilogy', but not in the quest for the key to the Slo-Time envelope of the planet Krikkit. The protagonists all completely fail to get the pieces of the key before the Krikkit robots appropriate them.
And so Slartibartfast and his companions are unable to prevent the reassembly of the key. Being in low spirits already that now the universe will come to an end violently as the Krikkit fighters are set free and stand good chances to realize their plan of destroying the whole universe with the help of the Ultimate Weapon, Arthur, Trillian, Ford and Slartibartfast make a final effort to avert the disaster. Trillian concludes that the whole development of Krikkit and all their weapon power was nurtured by Hactar. Since the planet was locked away from Hactar's influence by the envelope of Slo-Time, Trillian finds friends on the planet of Krikkit who help her and the others to get through to the leaders alive. After Trillian delivered a speech at the Elder Masters of Krikkit about her theory of Hactar bringing up the inhabitants of Krikkit as an extremely aggressive race she and Arthur factually meet Hactar, who welcomes them in the dust cloud which he, in fact, is. Trillian's theory is corroborated, Hactar planned to destroy the whole universe with his Ultimate Weapon as a revenge for his pulverization. ""Remember," he said, "that I was pulverized, and then left in a crippled and semi-impotent state for billions of years. I honestly would rather wipe out the universe. You would feel the same way, believe me."" (Life, chapter 32, p.145-146)
As soon as the facts have been found out, the decision is made to disperse Hactar and finally destroy his consciousness, which is carried out by The Heart of Gold. After that Slartibartfast and his Starship Bistromath vanish into "an entirely subjective idea of what space was" (Life, chapter 32, p.146) and are never mentioned again.
Arthur, Trillian, Ford and Zaphod are again on The Heart of Gold and eventually find Prak, a man who was a witness at a trial and who was given too much of a truth drug. When they find him he has already finished telling the "Truth, the Whole Truth and Nothing but the Truth" as he was told to do, but he can only remember very little. He seemingly knows much about Arthur Dent, but he must laugh about him persistently, so we do not get to know anything new. The one information that Prak gives us, is where they can find God's last message to his creation.
With this "Life, the Universe and Everything" ends. The sequel "So Long, and Thanks for all the Fish" is not tied to the previous plot. More specifically, it does not really fit into the concept of the Hitchhiker novels at all. This book does not carry on the plot of the previous story and even DNA never really liked this book. What happens is this: Arthur Dent returns to the place where the Earth should have been demolished. But he finds one, a shadow Earth which was put there by dolphins who initiated a Campaign to Save the Humans and left this replacement. All the people think the Vogons had been a mass hallucination caused by the CIA trying experiments in drug warfare. Two persons on Earth exist who seem to know more about it, or simply do not belong to this other Earth, Fenchurch and Wonko the Sane. Large parts of the book deal with a love story, which develops between Arthur and Fenchurch, who do both a lot of flying together after Arthur tought Fenchurch how it works. They visit Wonko the Sane who clears the situation up a bit by showing them a farewell gift from the dolphins.
Ford Prefect is the only other person from the previous novels who appears in this one: He is on his version of holiday in a war zone hitch-hiking. He takes control of one of the ships and directs it to Earth where he picks up Arthur and Fenchurch. The three of them then travel to the planet Prak reported God's last message on and meet Marvin, who is by this time thirty-seven times older than the universe due to time-travels. Marvin reads God's last message and dies. The message is "We apologise for the inconvenience".
"Mostly Harmless" marks the end of the Hitchhiker 'trilogy'. The book introduces another Earth in another parallel universe. There are differences between Arthur Dent's original home-planet and this Earth. It has never been demolished to make way for a hypergalactic bypass and clover has ordinarily four leaves and it is regarded as a sign of luck if you find one with three leaves and Tricia McMillan has never left this Earth. She has become a TV anchor person as she is absolutely sure that she missed her great chance by not leaving the party with Zaphod.
Another planet in the solar system is discovered, named Persephone and nicknamed Rupert after some astronomer's parrot. On this planet a damaged spaceship lands and monitors the Earth. This is because the damage affected the memory banks of the ship and all they can remember is that they should monitor something.
There are two other threads in "Mostly Harmless", Ford Prefect and Arthur Dent. Arthur has lost Fenchurch who has completely vanished during an accident in hyperspace, which is explained with the fact that Fenchurch originates from a plural sector of the galaxy, which is known as ZZ9 Plural Z Alpha, the position of the Earth. We learn that hyperspace travels are extremely dangerous for beings from sectors like this one, as their existence is not stable through all the different possible universes. Arthur tries to find a planet where he can settle down and lead a quiet life. He finds a planet, NowWhat, in the place where the Earth should have gone as it should have been demolished. It looks similar to the Earth he knows, but the only extant form of animal life on NowWhat are 'boghogs', "tiny vicious creatures" (Mostly Harmless, chapter 7, p.54) which communicate by biting each other very hard on the thigh. This is no planet Arthur feels at home on and he begins to hitch-hike through the galaxy financing the travels by donating body cells to DNA banks all over the universe.
Eventually one of the ships, on which he is on the way with, crashes onto the planet of Lamuella with him being the only survivor. The people of Lamuella are friendly and Arthur finds what he was searching for: He does the only thing he can do properly and becomes The Sandwich Maker of Lamuella and leads a quiet life until Trillian shows up and confronts him with their daughter Random. Trillian called her daughter Random as she went to a DNA bank for semen. As Arthur was the only existing homo sapiens donor Random is definitely his daughter. She leaves Random with Arthur as she (now working for one of the big Sub-Etha broadcasting networks) often has to travel in space and time, has to go to a war zone and is concerned about the safety of Random.
Ford Prefect finds out that the publishing coorporation of The Guide was bought by InfiniDim Enterprises which had immediately initiated major changes to the management of the marketing of The Guide. They also produced an entirely new Guide, The Hitch Hiker's Guide to the Galaxy Mk II, which Ford steals from their research department and sends to Arthur Dent in a parcel. Ford also finds out that InfiniDim Enterprises is partly an organization of Vogons, which alarms him and he soon concludes that something "is wildly, crazily, stupidly cross-eyed-blithering-insectly wrong" (Mostly Harmless, chapter 18, p.165) about their perception of all the things that happen. He explains to Arthur he meets on Lamuella how the new Guide works:
It makes use of the new technology of Unfiltered Perception. The new Guide can perceive everything. That means all parallel realities or all of the "Whole Sort of General Mish Mash" as DNA calls it. (Mostly Harmless, chapter 3, p.26). But as it is totally unlimited it actually changes the whole universe at free will, e.g. it enables Random who gets hold of it on Lamuella to visit an Earth which has not been destroyed by the Vogons. Shortly after she arrives there, Arthur and Ford get there, too. It is the Earth which was described at the beginning of the book with Tricia working as a TV anchor person who certainly does not know Random, although she is more or less the same person only on a different point in probability. Then Trillian appears as well and they all meet at a club named Beta, whose owner is a man called Stavro Mueller. A scuffle breaks out and Stavro Mueller is shot dead. The full name of the club is Stavro Mueller Beta and this is the expected event of the last killing of Agrajag. Shortly thereafter the Earth is destroyed by the Vogons, only that this time they make use of the Guide Mk II, which eradicates the existence of the Earth in all possible universes. This is quite definitely the end of the whole plot.