The front cover of the first U.S. hardcover edition of Mostly Harmless.
|Cover artist||Peter Cross, U.S. hardcover|
|Country||United Kingdom, United States|
|Series||The Hitchhiker's Guide to the Galaxy|
|Publisher||Pan Books, UK; Harmony Books, U.S.|
|Media type||Paperback, hardcover|
|Pages||229, UK paperback; 240, U.S. paperback|
|Preceded by||So Long, and Thanks for All the Fish|
|Followed by||And Another Thing...|
Mostly Harmless is a novel by Douglas Adams and the fifth book in the Hitchhiker's Guide to the Galaxy trilogy. It is described on the cover of the first editions as "The fifth book in the increasingly inaccurately named Hitchhikers Trilogy". It was the last Hitchhiker's book written by Adams.
The title derives from a joke early in the series, when Arthur Dent discovers that the entry for Earth in The Hitchhiker's Guide to the Galaxy consists, in its entirety, of the word "Harmless". His friend Ford Prefect, a contributor to the Guide, assures him that the next edition will contain the article on Earth that Ford has spent the last 15 years researching—somewhat cut due to space restrictions, but still an improvement. The revised article, he eventually admits, will simply read "Mostly harmless". It later turns out that Ford had written a long essay on how to have fun on Earth, but the editors in the guide's main office building edited everything out. Later in the series, Ford is surprised to discover that all of his contribution had been edited back into the Guide, prompting his reunion with Arthur on the alternative Earth in So Long, and Thanks for All the Fish.
Plot summary 
After the events in So Long, and Thanks for All the Fish, Arthur Dent and his love interest Fenchurch attempt to sightsee across the Galaxy, but when Fenchurch disappears during a hyperspace jump due to being from an unstable sector of the Galaxy, Arthur becomes depressed and travels the Galaxy alone, raising money to pay his passage by donating his biological material to DNA banks (mostly sperm, due to it having the highest payout). He is aware that he cannot die until he visits Stavromula Beta, as was revealed in Life, the Universe and Everything, by the insane Agrajag. During one journey, his ship crash lands on the planet Lamuella; Arthur survives and finds a simple life, becoming the sandwich maker for the local population.
Meanwhile, Ford Prefect has returned to the offices of the Hitchhikers' Guide, and is annoyed to find out the original publishing company, Megadodo Publications, has been taken over by InfiniDim Enterprises. Ford discovers that Vogons are part of the Guide's employ, and attempts to escape the building. During this attempt, he finds the Hitchhiker's Guide Mk. II, which he steals and sends to himself, care of Arthur, for safekeeping. Arthur, as Ford had expected, stores the package away without even opening it and forgets about it.
One day, Trillian arrives on Lamuella, and presents Arthur with Random Dent, a teenage girl she claims is his daughter, conceived by Trillian via artificial insemination using the only human sperm samples available, those given by Arthur to pay for his space travels. Trillian leaves Random with Arthur so that she can better pursue her new career as an intergalactic reporter. Random finds life on Lamuella boring and cannot get along with Arthur, and comes across the Guide Mk. II. With Random as its owner, the Guide helps her to escape the planet to find her mother. Ford appears shortly after, looking for the Guide. Together, Ford and Arthur manage to leave the planet and return to Earth, realizing that Random has also returned there. Ford also realizes that the Guide Mk. II, being capable of both time-travel and looking into alternate universes, is manipulating events in accordance with an unknown plan.
Meanwhile, on Earth, an alternative version of Trillian, reporter Tricia McMillan, who never was able to take Zaphod Beeblebrox's offer to travel in space due to wanting to get her handbag, finds herself approached by an extraterrestrial species calling themselves the Grebulons. They reveal that they have set up a base on Rupert (a tenth planet just beyond the orbit of Pluto) after arriving in the Solar System and finding that their computer core and most of their memories had been damaged, and have been following the remaining portions of their mission statement to observe the most interesting things in the system. They have approached Tricia to help them adapt the astrology charts to use Rupert as their base, offering to let her interview them on their base in exchange for the help. Tricia performs the interview but finds that the results look questionable. She is then called to report on a spaceship landing in the middle of London.
Tricia finds Random leaving the ship, and the girl believes Tricia to be her mother, and starts yelling at her. Tricia, with the help of Arthur, Ford, and Trillian, manage to take Random to a bar (address number 42) to try to talk to her. However, she fires a weapon at Arthur; Arthur ducks and the shot kills a man behind him. Ford points out to Arthur that the name of the bar they're in is "Stavro Mueller's Beta", the man just killed being another form of Agrajag, and that Arthur's life is no longer safe, a point at which Arthur takes relief. Shortly afterward, the Grebulons, believing that it would be a positive action to improve their situation, fire upon and destroy the Earth.
It is revealed that the Guide Mk. II was created by the Vogons to complete the destruction of the Earth in every possible dimension, using reverse temporal engineering to bring Arthur, Ford, Trillian, and Random together on Earth for its destruction by the Vogons. After the Earth is destroyed, the Guide collapses in on itself.
In the alternate ending, after the destruction of Earth, the description of the Babel fish from the earlier series is replayed with an additional section, which states that dolphins and Babel fish are acquainted, and that the dolphins' ability to travel through possibility space (first mentioned in The Hitchhiker's Guide to the Galaxy (novel) and elaborated on in So Long, and Thanks for All the Fish) is shared by the Babel fish as well. During the ending, Ford explains that the dolphins got taught this skill from the Babel fish in exchange for knowing a good place to have parties. All the major characters are carrying Babel fish in their ears, which rescue them at the moment of Earth's destruction by transporting them to the Restaurant at the End of the Universe. The characters are reunited with Marvin, and it is revealed that beyond the Restaurant (and beyond the car park in which Marvin works) lies an endless series of blue lagoons — the final destination of the dolphins. The series ends with Arthur asking Fenchurch, "Will you come flying with me?", and her reply, "Always."
The version released on CD contains an even longer set of alternate endings, including one set after the events of the twelfth radio episode (with Arthur Dent and Lintilla), and on an alternate Earth where Arthur Dent and Fenchurch engage in a stand-off against Mr Prosser, together.
There have been two unabridged audiobook recordings of the novel. In 1992, Adams himself recorded an edition, later re-released by New Millennium Audio in the United States and available from BBC Audiobooks in the United Kingdom. In 2006, actor Martin Freeman, who had played Arthur Dent in the 2005 movie, recorded a new edition of the audiobook. This is the only book in the five novel series not to have also had a prior, abridged edition read by Stephen Moore.
Academic and popular culture 
Joshua D. Angrist and Joern-Steffen Pischke named their applied econometrics toolkit book "Mostly Harmless Econometrics" in the spirit of Douglas Adams' The Hitchhiker's Guide to the Galaxy and Mostly Harmless.
- Angrist, Joshua D., Pischke, Joern-Steffen (2009). Mostly Harmless Econometrics, An Empiricist's Compation. Princeton and Oxford: Princeton University Press.