The starship Heart of Gold is headed for the planet Magrathea, a planet which it is generally agreed does not exist. In trying to land on Magrathea's surface, the crew faces an ancient nuclear missile defence system, escaping only when Arthur turns on the Infinite Improbability Drive. This helpfully replaces the missiles with a bowl of petunias and a very surprised sperm whale, who learns about the ground before he hits it. Later on, deep in the core of the planet, an unknown enemy attacks Trillian, Zaphod and Ford while Arthur meets Slartibartfast, an old man who designs planets for a living. Slartibartfast takes Arthur on a tour of the factory floor, showing off his latest project - Planet Earth, Mark Two.

Arthur learns about the Great Project - the second most powerful computer in existence, called Deep Thought, created to answer the Ultimate Question of Life, the Universe and Everything. The computer comes back online much later on to give its final answer - 42.

The scientists are understandably hacked off, even more so when Deep Thought cannot tell them what the Ultimate Question is, and a new computer, called the Earth, has to be built for that purpose.

The current owners of the Great Project, Trillian's pet mice, want to cut open Arthur's head to find the Question, but when the crew are all trapped behind a bank of exploding computers, all appears lost.