The novels are described as "a trilogy in five parts", having been described as a trilogy on the release of the third book, and then a "trilogy in four parts" on the release of the fourth book. The US edition of the fifth book was originally released with the legend "The fifth book in the increasingly inaccurately named Hitchhiker's Trilogy" on the cover. Subsequent re-releases of the other novels bore the legend "The [first, second, third, fourth] book in the increasingly inaccurately named Hitchhiker's Trilogy". In addition, the blurb on the fifth book describes it as "the book that gives a whole new meaning to the word 'trilogy'".
The plots of the television and radio series are more or less the same as that of the first two novels, though some of the events occur in a different order and many of the details are changed. Much of parts five and six of the radio series were written by John Lloyd, but his material did not make it into the other versions of the story and is not included here. Many consider the books' version of events to be definitive because they are the most readily accessible and widely distributed version of the story. However, they are not the final version that Adams produced.
Before his death from a heart attack on 11 May 2001, Adams was considering writing a sixth novel in the Hitchhiker's series. He was working on a third Dirk Gently novel, under the working title The Salmon of Doubt, but felt that the book was not working and abandoned it. In an interview, he said some of the ideas in the book might fit better in the Hitchhiker's series, and suggested he might rework those ideas into a sixth book in that series. He described Mostly Harmless as "a very bleak book" and said he "would love to finish Hitchhiker on a slightly more upbeat note". Adams also remarked that if he were to write a sixth instalment, he would at least start with all the characters in the same place. Eoin Colfer, who wrote the sixth book in the Hitchhiker's series in 2008–09, used this latter concept but none of the plot ideas from The Salmon of Doubt.
The Hitchhiker's Guide to the Galaxy
In The Hitchhiker's Guide to the Galaxy (published in 1979), the characters visit the legendary planet Magrathea, home to the now-collapsed planet-building industry, and meet Slartibartfast, a planetary coastline designer who was responsible for the fjords of Norway. Through archival recordings, he relates the story of a race of hyper-intelligent pan-dimensional beings who built a computer named Deep Thought to calculate the Answer to the Ultimate Question of Life, the Universe, and Everything. When the answer was revealed to be 42, Deep Thought explained that the answer was incomprehensible because the beings didn't know what they were asking. It went on to predict that another computer, more powerful than itself would be made and designed by it to calculate the question for the answer. (Later on, referencing this, Adams would create the 42 Puzzle, a puzzle which could be approached in multiple ways, all yielding the answer 42.)
The computer, often mistaken for a planet (because of its size and use of biological components), was the Earth, and was destroyed by Vogons to make way for a hyperspatial express route five minutes before the conclusion of its 10-million-year program. Two members of the race of hyper-intelligent pan-dimensional beings who commissioned the Earth in the first place disguise themselves as Trillian's mice, and want to dissect Arthur's brain to help reconstruct the question, since he was part of the Earth's matrix moments before it was destroyed, and so he is likely to have part of the question buried in his brain. Trillian is also human but had left Earth six months previously with Zaphod Beeblebrox, President of the Galaxy. The protagonists escape, setting the course for "The Restaurant at the End of the Universe". The mice, in Arthur's absence, create a phony question since it is too troublesome for them to wait 10 million years again just to cash in on a lucrative deal.
The book was adapted from the first four radio episodes. It was first published in 1979, initially in paperback, by Pan Books, after BBC Publishing had turned down the offer of publishing a novelization, an action they would later regret. The book reached number one on the book charts in only its second week, and sold over 250,000 copies within three months of its release. A hardback edition was published by Harmony Books, a division of Random House in the United States in October 1980, and the 1981 US paperback edition was promoted by the give-away of 3,000 free copies in the magazine Rolling Stone to build word of mouth. In 2005, Del Rey Books rereleased the Hitchhiker series with new covers for the release of the 2005 movie. To date, it has sold over 14 million copies.
A photo-illustrated edition of the first novel appeared in 1994.
The Restaurant at the End of the Universe
In The Restaurant at the End of the Universe (published in 1980), Zaphod is separated from the others and finds he is part of a conspiracy to uncover who really runs the Universe. Zaphod meets Zarniwoop, a conspirator and editor for The Guide, who knows where to find the secret ruler. Zaphod becomes briefly reunited with the others for a trip to Milliways, the restaurant of the title. Zaphod and Ford decide to steal a ship from there, which turns out to be a stunt ship pre-programmed to plunge into a star as a special effect in a stage show. Unable to change course, the main characters get Marvin to run the teleporter they find in the ship, which is working other than having no automatic control (someone must remain behind to operate it), and Marvin seemingly sacrifices himself. Zaphod and Trillian discover that the Universe is in the safe hands of a simple man living on a remote planet in a wooden shack with his cat.
Ford and Arthur, meanwhile, end up on a spacecraft full of the outcasts of the Golgafrinchan civilization. The ship crashes on prehistoric Earth; Ford and Arthur are stranded, and it becomes clear that the inept Golgafrinchans are the ancestors of modern humans, having displaced the Earth's indigenous hominids. This has disrupted the Earth's programming so that when Ford and Arthur manage to extract the final readout from Arthur's subconscious mind by pulling lettered tiles from a Scrabble set, it is "What do you get if you multiply six by nine?" Arthur then comments, "I've always said there was something fundamentally wrong with the universe."
The book was adapted from the remaining material in the radio series—covering from the fifth episode to the twelfth episode, although the ordering was greatly changed (in particular, the events of Fit the Sixth, with Ford and Arthur being stranded on pre-historic Earth, end the book, and their rescue in Fit the Seventh is deleted), and most of the Brontitall incident was omitted, instead of the Haggunenon sequence, co-written by John Loyd, the Disaster Area stunt ship was substituted—this having first been introduced in the LP version. Adams himself considered Restaurant to be his best novel of the five.
Life, the Universe and Everything
In Life, the Universe and Everything (published in 1982), Ford and Arthur travel through the space-time continuum from prehistoric Earth to Lord's Cricket Ground. There they run into Slartibartfast, who enlists their aid in preventing galactic war. Long ago, the people of Krikkit attempted to wipe out all life in the Universe, but they were stopped and imprisoned on their home planet; now they are poised to escape. With the help of Marvin, Zaphod, and Trillian, our heroes prevent the destruction of life in the Universe and go their separate ways.
This was the first Hitchhiker's book originally written as a book and not adapted from radio. Its story was based on a treatment Adams had written for a Doctor Who theatrical release, with the Doctor role being split between Slartibartfast (to begin with), and later Trillian and Arthur.
In 2004 it was adapted for radio as the Tertiary Phase of the radio series.
So Long, and Thanks for All the Fish
In So Long, and Thanks for All the Fish (published in 1984), Arthur returns home to Earth, rather surprisingly since it was destroyed when he left. He meets and falls in love with a girl named Fenchurch, and discovers this Earth is a replacement provided by the dolphins in their Save the Humans campaign. Eventually, he rejoins Ford, who claims to have saved the Universe in the meantime, to hitch-hike one last time and see God's Final Message to His Creation. Along the way, they are joined by Marvin, the Paranoid Android, who, although 37 times older than the universe itself (what with time travel and all), has just enough power left in his failing body to read the message and feel better about it all before expiring.
This was the first Hitchhiker's novel which was not an adaptation of any previously written story or script. In 2005 it was adapted for radio as the Quandary Phase of the radio series.
Finally, in Mostly Harmless (published in 1992), Vogons take over The Hitchhiker's Guide (under the name of InfiniDim Enterprises), to finish, once and for all, the task of obliterating the Earth. After abruptly losing Fenchurch and traveling around the galaxy despondently, Arthur's spaceship crashes on the planet Lamuella, where he settles in happily as the official sandwich-maker for a small village of simple, peaceful people. Meanwhile, Ford Prefect breaks into The Guide's offices, gets himself an infinite expense account from the computer system, and then meets The Hitchhiker's Guide to the Galaxy, Mark II, an artificially intelligent, multi-dimensional guide with vast power and a hidden purpose. After he declines this dangerously powerful machine's aid (which he receives anyway), he sends it to Arthur Dent for safety ("Oh yes, whose?"—Arthur).
Trillian uses DNA that Arthur donated for traveling money to have a daughter, and when she goes to cover a war, she leaves her daughter Random Frequent Flyer Dent with Arthur. Random, a more than typically troubled teenager, steals The Guide Mark II and uses it to get to Earth. Arthur, Ford, Trillian, and Tricia McMillan (Trillian in this alternate universe) follow her to a crowded club, where an anguished Random becomes startled by a noise and inadvertently fires her gun at Arthur. The shot misses Arthur and kills a man (the ever-unfortunate Agrajag). Immediately afterwards, The Guide Mark II causes the removal of all possible Earths from probability. All of the main characters, save Zaphod, were on Earth at the time and are apparently killed, bringing a good deal of satisfaction to the Vogons.
In 2005 it was adapted for radio as the Quintessential Phase of the radio series, with the final episode first transmitted on 21 June 2005.
And Another Thing...
It was announced in September 2008 that Eoin Colfer, author of Artemis Fowl, had been commissioned to write the sixth instalment entitled And Another Thing... with the support of Jane Belson, Adams's widow. The book was published by Penguin Books in the UK and Hyperion in the US in October 2009.
The story begins as death rays bear down on Earth, and the characters awaken from a virtual reality. Zaphod picks them up shortly before they are killed, but completely fails to escape the death beams. They are then saved by Bowerick Wowbagger, the Infinitely Prolonged, whom they agree to help kill. Zaphod travels to Asgard to get Thor's help. In the meantime, the Vogons are heading to destroy a colony of people who also escaped Earth's destruction, on the planet Nano. Arthur, Wowbagger, Trillian and Random head to Nano to try to stop the Vogons, and on the journey, Wowbagger and Trillian fall in love, making Wowbagger question whether or not he wants to be killed. Zaphod arrives with Thor, who then signs up to be the planet's God. With Random's help, Thor almost kills Wowbagger. Wowbagger, who merely loses his immortality, then marries Trillian.
Thor then stops the first Vogon attack and apparently dies. Meanwhile, Constant Mown, son of Prostetnic Jeltz, convinces his father that the people on the planet are not citizens of Earth, but are, in fact, citizens of Nano, which means that it would be illegal to kill them. As the book draws to a close, Arthur is on his way to check out a possible university for Random, when, during a hyperspace jump, he is flung across alternate universes, has a brief encounter with Fenchurch, and ends up exactly where he would want to be. And then the Vogons turn up again.
In 2017 it was adapted for radio as the Hexagonal Phase of the radio series, with its premiere episode first transmitted on 8 March 2018 (exactly forty years, to the day, from the first episode of the first series, the Primary Phase).