Arthur Dent, a perfectly ordinary Earthman, is surprised to wake up one day to find bulldozers outside his house with orders to knock it down to build a by-pass. He is even more surprised later on in the pub when his best friend Ford Prefect reveals himself to be from a small planet somewhere in the vicinity of Betelgeuse. The two are forced to hitch a lift on one of the advancing Vogon spacecraft which proceed to blow up the Earth to make way for an interspace bypass. Our two heroes find themselves trapped in a storage room in hyperspace, with only a menacing Vogon guard for company.

Arthur and Ford have been discovered. Vogon Captain Jeltz tortures them by reading his poetry. He then has Arthur and Ford thrown off his ship, to what must be certain death - except for one improbable miracle. At the last second, an infinite-improbability prototype ship (which can pass through every point in the Universe) rescues them. What is rather surprising is that Zaphod Beeblebrox, the hip cat who stole the spaceship, is vaguely familiar to Arthur. And so is Zaphod's companion, Trillian.

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.

Arthur, Ford and the gang arrive at the Restaurant at the End of the Universe on Magrathea, built on the ruins of a planet-building complex, transported there by the exploding computers. Here, thanks to compound interest, you can enjoy a vast five-course meal and cabaret at no cost whatever while the universe collapses and dies around you. Arthur and Ford are surprised to find Marvin still waiting for them in the restaurant's star-ship park. He has been there rather a long time. They decide to steal a space-ship, but unfortunately it turns out to be the stunt ship of Disaster Area, the loudest rock band of all time, and is programmed to plunge directly into the sun. So what next?

Fortune is on the crews' side - the transporter on the ship is still working, but needs someone to manually operate it. Marvin is therefore pressed into volunteering to sacrifice himself so the rest of the group can escape. Arthur and Ford get separated from Zaphod and Trillian and find themselves on board a space ship about to crashland into the prehistoric past of a planet that the two of them find strangely familiar. If this is indeed Earth, then history as they know it is about to be changed - and so is the Ultimate Question to Life, the Universe and Everything....