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 civilisation. 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.

