After being improbably rescued from certain death in the vacuum of space, Arthur Dent and his new companions now face a missile attack and certain death.

Zaphod explains that he stole the Heart of Gold so that he could fly to the legendary planet of Magrathea, which is noted for being the planet that made a vast fortune making other planets.

Ford believes that Magrathea does not exist, but is soon proved wrong when not only do they discover the planet, but they are attacked by two nuclear missiles. Luckily, Arthur saves everyone using the Infinite Improbability Drive, resulting in the missiles turning into a bowl of petunias and a very surprised-looking sperm whale. Trillian also discovers that her white mice have escaped.

When they land on the planet, Ford, Zaphod and Trillian go down a tunnel and end up being attacked. Arthur is left with Marvin the Paranoid Android on the surface of the planet, where they meet a native, Slartibartfast. Arthur and Slartibartfast travel deep inside the planet, where Arthur makes a disturbing discovery: Earth was built by Magrathea in order to discover something called "The Ultimate Question".