The Book: Peter Jones Arthur Dent: Simon Jones Slartibartfast: Richard Vernon Deep Thought and Ford Prefect: Geoffrey McGivern Zaphod Beeblebrox: Mark Wing-Davey Trillian: Susan Sheridan First Computer Programmer/Bang Bang/Magrathean PA Voice: Ray Hassett Second Computer Programmer: Jeremy SR Browne Cheerleader and Majikthise: Jonathan Adams Vroomfondel and Shooty: James Broadbent Frankie Mouse: Peter Hawkins Benjy Mouse: David Tate Announcer (uncredited): John Marsh (introductory announcement only)
The episode begins with a recap of the events of the series so far, before moving to a conversation where Slartibartfast explains that mice are really "the protrusions into our dimension of vast hyper-intelligent pan-dimensional beings", and that they commissioned the Earth to be built. He plays Arthur some recordings explaining the historical events. This race of pan-dimensional beings had constructed a great computer, called Deep Thought, to answer the Ultimate Question of Life, the Universe and Everything. It did, after seven and a half million years, have the answer to the Ultimate Question, a rather disappointing 42. Deep Thought explains that this is only disappointing because they never really understood what the Question was. They ask the computer if he can find out what the Ultimate Question is. Deep Thought cannot, but promises to design a computer that can, and names it, "Earth".
Slartibartfast explains that this computer was built by the Magratheans, and that the Vogons came and destroyed it five minutes before it was due to complete its run. The mice summon Arthur and Slartibartfast to a meeting room, where they have discussed a proposal with Zaphod, Ford and Trillian. The mice believe that as last-generation products of the computer matrix, Arthur and Trillian should be in an ideal position to find out the Question, and offer to make them "extremely rich" if they can do so. (In later versions this would be replaced with the mice wishing to extract Arthur's brain). The negotiations are interrupted by the arrival of a Galactic Police ship, pursuing Zaphod for his theft of the Heart of Gold.
The Police confront Arthur, Ford, Trillian and Zaphod, and shoot at them, whilst explaining that they find violence upsetting. After a particularly long volley of fire, the computer bank they are hiding behind explodes, and the episode ends.
Music: A Rainbow in Curved Air by Terry Riley; Moon City from In Search of Ancient Gods by Absolute Elsewhere; Mikrophonie I by Stockhausen.