The story opens by shrinking Earth to an almost negligible spot in the Galaxy. Its inhabitants are unhappy much of the time and keep proposing solutions involving money, although money is not what is suffering. One woman finally sees how the world might be made happy, but a catastrophe prevents her from telling anyone. The novel follows that catastrophe and some of its consequences. It also follows the Hitchhiker’s Guide to the Galaxy, a celebrated book with gaps and unreliable entries. It is cheaper than its more solemn rival and carries one immediately usable instruction on its cover: don’t panic.

The first demolition is small enough to have a bulldozer parked outside Arthur Dent’s house. A bypass is about to replace his home, and Arthur lies in the mud to stop the work. The council says the plans have been available for nine months. He found them in a locked filing cabinet inside a disused lavatory. The procedure has technically been followed, while the person most affected has had no meaningful chance to respond. Mr Prosser can offer no better reason for building the bypass than the fact that bypasses are the sort of thing one builds.

Ford Prefect arrives to collect Arthur. Arthur thinks his friend comes from Guildford; Ford is actually a travelling researcher for the Guide, stranded on Earth for fifteen years after arriving from a planet near Betelgeuse. Ford persuades Mr Prosser to take Arthur’s place in front of the bulldozer and takes Arthur to the pub. He knows that ships are approaching and urges Arthur to drink. While Arthur discovers that his house has been knocked down after all, the Vogons announce a second project: Earth must be removed for a hyperspace express route.

The notice for that demolition, they say, has long been displayed in a planning office on Alpha Centauri. Their excuse repeats the council’s logic on a scale at which no human protest can stop the work. Earth is destroyed. Ford and Arthur survive by transferring aboard a Vogon ship, helped by the Dentrassis who work there as cooks. Arthur has lost both his house and his planet in a single day. The change in scale is funny, but it does not make the loss unreal to him.

Aboard the Vogon ship, Ford gives Arthur food to help him recover from the transfer and shows him the electronic Guide. Its entry for Earth originally consists of one word, “Harmless”. Ford has managed to improve it to “Mostly harmless”, a painfully small tribute to the world Arthur has just lost. Ford also puts a Babel fish in Arthur’s ear so he can understand alien speech. The Guide offers a biological account of its translation, a joke about proving God’s nonexistence, and a darker observation: removing language barriers has not prevented wars between cultures.

The Vogon captain discovers the stowaways. Before expelling them, he makes them hear his poetry through equipment designed to intensify every detail. Arthur tries to save them by praising the poem, and Ford joins in with improvised literary commentary. It fails. As a guard drags them toward the airlock, Ford asks whether shouting at people and throwing them out of ships makes for a satisfying life. The guard admits that most of his working minutes are poor, yet promotion depends on continuing to shout and push. He carries out the order and ejects them into space.

The Guide says they can survive about thirty seconds in a vacuum, while judging rescue within that time fantastically unlikely. A ship picks them up after twenty-nine seconds. Its drive briefly makes their surroundings behave in impossible ways before ordinary probability returns. This is the Heart of Gold, powered by the newly developed Infinite Improbability Drive. Zaphod Beeblebrox, a Galactic President whose office mainly diverts attention from power elsewhere, stole the prototype at its unveiling. Trillian looks after the ship, and Marvin, a brilliant, miserable robot, brings the survivors to its bridge.

The meeting gathers several histories into one improbable room. Ford knows Zaphod as his semi-cousin. Arthur recognizes him as the man who, calling himself Phil, once interrupted Arthur’s conversation with a woman at a party on Earth. That woman is Trillian, who accepted Zaphod’s offer of a lift away from the planet. Their reunion is startling even to people travelling by improbable means. The ship itself picked Ford and Arthur up during its flight. Zaphod is nevertheless pursuing a destination of his own: Magrathea, a legendary planet hidden in the Horsehead Nebula.

Magrathea once built planets to order. Its wealthy customers could buy a world shaped around their preferences, down to the colour of its seas or the length of its days. Ford initially doubts that such a business ever existed. Zaphod insists that he has found its home, though he cannot clearly explain what he is seeking there. From orbit the planet looks dead, but an old automatic system warns the ship away and fires two missiles. Nobody aboard can manage the manual controls. Arthur switches on the Improbability Drive without programming it, turning the missiles into a bowl of petunias and a sperm whale.

The ship lands. The whale, brought into existence only moments before, has fallen to the surface and opened a passage into the planet. Zaphod leads Ford and Trillian below ground while Arthur remains outside with Marvin. In the passages, Zaphod admits to Ford that he does not fully understand his own decisions. He became president, stole the ship and sought Magrathea, yet suspects that an inaccessible part of his mind has been steering him. A steel shutter and a release of gas interrupt the conversation. At this stage, the precise reason for his search remains unexplained.

Arthur meets Slartibartfast, an elderly Magrathean, on the surface. He says his people slept for five million years after an economic collapse left no market for custom-built luxury planets. He takes Arthur into a vast factory housed in hyperspace and reveals that Earth was one of their products. Slartibartfast designed its Norwegian fjords and remembers them with pride. Earth’s clients, he says, were mice. The planet was destroyed five minutes before it completed the purpose for which they had commissioned it; work on a replacement Earth has already begun.

The explanation reverses several human assumptions. According to Slartibartfast, the mice seen in laboratories were only the visible presence of hyperintelligent beings from another dimension. Their behaviour in experiments was guiding the human researchers who thought they were studying them. The story has already revealed that dolphins knew Earth would be destroyed and tried to warn humanity, which mistook their messages for entertaining tricks. Arthur now learns that humans and their planet formed the matrix of an organic computer carrying out a ten-million-year research programme.

Slartibartfast takes Arthur back to the programme’s origins. Two representatives of an earlier age activate Deep Thought, a computer built to answer the great question of life, the Universe and everything. It calls itself the second greatest computer there will be: a greater one is to follow it. Two philosophers immediately object. They fear that a definite answer will put them out of work and claim the right to keep debating uncertain truths. Deep Thought continues its calculation regardless. It announces that the result will take seven and a half million years.

Arthur then experiences a Magrathean recording of the day the programmers’ descendants receive the result. The ceremony and the long wait are out of proportion to the answer’s apparent simplicity. The supplied editorial notice identifies that answer as “42” and states that the question needed to make sense of it remains unknown. The number is comic because it formally answers a vast request while explaining nothing about what was really asked. Computational power cannot make an undefined question precise.

Earth thus has a particular place in the story. It was Arthur’s lost home, but it was also a device intended to carry a search beyond the first answer. That purpose offers him no consolation. Slartibartfast mourns a destroyed piece of workmanship, especially its coastlines; the clients have lost a programme interrupted just before completion; Arthur remembers a place where he lived. Their losses overlap without becoming the same loss. The novel repeatedly brings immense galactic explanations back to ordinary needs, habits and grief.

Its jokes work through that movement too. Demolition notices placed beyond the reach of those affected, a useful but unreliable Guide, a robot burdened with a personality he hates, and a ship saved by Arthur’s desperate guess all show systems that operate while failing the people who depend on them. The supplied source evidence stops as Deep Thought is about to give its answer. The editorial notice establishes the number 42, but the packet does not document the rest of that scene or the expedition’s ending on Magrathea. What it does establish is the tension between possessing an answer and losing the means to discover its question.