"Life, the Universe, and Everything" is a humorous science fiction novel that tells the story of the accidental space traveler Arthur Dent, an Englishman who escapes the destruction of the Earth and ultimately saves the universe from complete destruction. The book is the third in the series of "Hitchhiker's Guide to the Galaxy" books.

As the book opens, Arthur Dent is living in a cave on prehistoric Earth having been sent back in time at the end of the previous book in the series. He lives alone and is almost crazy when his former traveling companion, Ford Prefect, who has been living on another part of the planet, appears suddenly. The two of them are whisked through time and space to a future Earth, where they suddenly materialize on a velvet paisley couch in the middle of a cricket match in England just days before the Earth is to be destroyed. The match is interrupted when a spaceship full of white-clad robots arrives, steals the cricket trophy that has just been won by England, and begins killing everyone.

A figure named Slartibartfast, who commands a spaceship that resembles an Italian bistro, rescues Arthur and Ford. A strange form of mathematics based on the complex science of eating at a restaurant and dividing the bill, powers the spaceship. Arthur and Ford reluctantly join in Slartibartfast's mission to stop the killer robots from freeing the imprisoned planet of Krikkit, whose inhabitants have been frozen in time as punishment for their campaign to destroy the entire universe. The robots have been programmed to scour the universe for the various parts of a certain key, which will unlock the time warp in which Krikkit has been enveloped.

After attending a cocktail party that has been going on for centuries, Arthur, Ford and Slartibartfast team up with Zaphod Beeblebrox, a vain two-headed alien with a powerful spacecraft, and his sometimes girlfriend, Trillian, to trace the robots back to a giant supercomputer which has manipulated the people of Krikkit into their murderous campaign and which has built a small round bomb that will allow Krikkit to cause every star in the universe to explode. Trillian is able to convince the people of Krikkit not to destroy the universe, however Arthur Dent very nearly sets off the bomb, mistaking it for a cricket ball. He realizes his error just in time to stop it from going off.

Interspersed throughout the books are short, humorous episodes involving bizarre creatures and unlikely locations as well as passages from the "Hitchhiker's Guide to the Galaxy," an electronic book that guides casual travelers through the universe.