The story begins with a prologue that introduces the
Hitchhiker's Guide to the Galaxy, an electronic book that contains information
about every planet in the galaxy and describes humans as a primitive and
unhappy species. Arthur Dent, an Englishman and an Earthman, wakes up in his
house in the West Country and finds out that the local council wants to knock
down his house to build a bypass. He tries to stop them by lying in front of
the bulldozer.
His friend Ford Prefect, who is actually an alien from a
planet near Betelgeuse and a researcher for the Hitchhiker's Guide to the
Galaxy, persuades the chief official to take Arthur's place and takes Arthur to
the pub. The workers continue to destroy the house, but they are interrupted by
a fleet of spaceships that appear in the sky. The spaceships belong to the
Vogons, a heartless race of bureaucrats who announce that they are going to
blow up Earth to make room for a hyperspace highway, and they do so.
Ford and Arthur manage to escape by hitching a ride on one
of the spaceships, which astonishes Arthur. Ford tells Arthur that he has been
living on Earth as an unemployed actor from Guildford for 15 years, and that's
how he knew how to hitch a ride on the alien ship. They are soon discovered by
the Vogons, who make them listen to their awful poetry and then throw them out
of the airlock.
On the other side of the galaxy, Zaphod Beeblebrox, the
President of the Galaxy and Ford's "semi-cousin", steals a spaceship
called the Heart of Gold at its launch with his human friend, Trillian. The
Heart of Gold has a special device called the "Infinite Improbability
Drive" that allows it to travel anywhere in space by passing through every
point in the universe at once. However, the Infinite Improbability Drive also
causes impossible things to happen in the physical universe. One of these
things is that Arthur and Ford are rescued by the Heart of Gold as it uses the
Infinite Improbability Drive.
Zaphod takes his passengers — Arthur, Ford, Trillian, and a
depressed robot named Marvin — to a mythical planet called Magrathea. The
planet was famous for making custom-made planets for rich clients, but it
disappeared after causing an economic collapse in the galaxy. Ford is skeptical
that the planet is really Magrathea, but the planet's computers warn them to go
away before launching two nuclear missiles at the Heart of Gold. Arthur
accidentally saves them by activating the Infinite Improbability Drive incorrectly,
which also creates an underground tunnel. The ship lands on the planet, and
Trillian's pet mice Frankie and Benjy run away.
Zaphod, Ford, and Trillian go down to the core of Magrathea,
leaving Arthur and Marvin on the surface. Zaphod tells them that he has been
following a hidden plan that he himself set up but can't remember. As he
explains how he found out, they are caught and knocked out by sleeping gas.
Arthur meets a Magrathean named Slartibartfast, who tells him that the
Magratheans have been sleeping for a long time because of a financial crisis.
They have woken up to make a new Earth for the mice, who are actually the smartest
creatures on Earth.
Slartibartfast shows Arthur around the planet factory, and
tells him that a long time ago, a group of "hyperintelligent,
pan-dimensional beings" built a supercomputer called Deep Thought to find
out the answer to the "Ultimate Question to Life, the Universe, and
Everything." Deep Thought gave the answer as 42, but nobody knew what the
question was. So Deep Thought designed another supercomputer that could find
out the question. This supercomputer was Earth, which was made by the Magratheans,
and it was almost done with its job when the Vogons blew it up. The
hyperintelligent beings took part in the program as mice, pretending to be lab
animals while studying humans.
Slartibartfast takes Arthur to meet his friends, who are
having a feast with Trillian's pet mice. The mice say that they don't need a
new Earth anymore, because they think that Arthur's brain has the Ultimate
Question. They offer to buy Arthur's brain, but he refuses and they start
fighting. They escape when the planet's alarm goes off, but they run into some
police who are after Zaphod.
The police trap Zaphod, Arthur, Ford and Trillian behind a
computer that is about to blow up from their guns. But then the police die
because their life-support systems fail. Ford finds out that Marvin talked to
their spaceship and made it kill itself because of his pessimism. The five get
away from Magrathea and decide to go to The Restaurant at the End of the
Universe.