The Demolition of Earth
Arthur Dent, an ordinary Englishman, wakes to find his house about to be demolished for a bypass. Before he can fully process this, his friend Ford Prefect reveals he is an alien from Betelgeuse and a writer for 'The Hitchhiker's Guide to the Galaxy'. Ford explains that the Vogons, a notoriously bureaucratic and unpleasant alien race, are about to demolish Earth for a hyperspace bypass. Despite Arthur's protests, the Vogons arrive, and Earth is destroyed. Ford saves himself and Arthur by hitchhiking onto a Vogon constructor fleet ship just seconds before the planet's destruction, starting their involuntary journey through the galaxy.
Escape from the Vogon Ship and the Heart of Gold
After being found by the Vogon captain, Prostetnic Vogon Jeltz, Arthur and Ford are forced to listen to awful Vogon poetry, then thrown into space to die. By chance, they are picked up by the 'Heart of Gold', a spaceship with the Infinite Improbability Drive, stolen by Ford's semi-cousin, Zaphod Beeblebrox, the two-headed, three-armed ex-President of the Galaxy. On board, they also find Trillian (Tricia McMillan), an astrophysicist Arthur once met, and Marvin, a very depressed robot. Zaphod is on a mysterious quest, driven by a deep urge he does not understand.
The Search for Magrathea
Zaphod's quest leads them to Magrathea, a planet fabled to have built planets for the wealthy. Despite warnings from the planet's automated defense system, they land. Inside, they find a large, dormant planet-building industry. They meet Slartibartfast, an old Magrathean designer, who reveals that Earth was not destroyed for a bypass. Instead, it was a supercomputer built by the Magratheans for a highly intelligent pan-dimensional race (mice) to find the Ultimate Question to the Answer of Life, the Universe, and Everything, which is 42.
The Restaurant at the End of the Universe
After escaping the Magratheans and the angry mice, the crew travels to Milliways, the Restaurant at the End of the Universe, to see the final moments of creation. Here, a group of intelligent, but doomed, marketing executives from Squornshellous Zeta pursue them, believing Zaphod holds the key to their salvation. They also meet Hotblack Desiato and his bored bodyguard, Zarniwoop. The group gets separated, and Arthur and Ford are stranded on a prehistoric Earth after an accident with the Infinite Improbability Drive.
Life, the Universe and Everything: Krikkit and the Supercomputer
Arthur, Ford, and Slartibartfast are recruited to stop the Krikkitmen, a xenophobic race who once decided to destroy the universe, from completing their old plan. They learn that the Krikkitmen were trapped in a 'fast time bubble' but are about to escape. The protagonists must get the 'Wikkit Gate', a crucial part, to prevent the Krikkitmen from activating their doomsday weapon. This involves a journey through strange places and meetings with unusual characters, eventually leading them back to Earth's ruins.
The Return to Earth and the Grebulons
Arthur unexpectedly finds himself back on a seemingly intact Earth, eight years after its supposed destruction. He tries to resume a normal life, but strange things, like a sentient fishbowl, suggest something is wrong. He soon discovers that this Earth is a copy, made by the Grebulons, a dim-witted alien race who mistake him for an expert on Earth and its mysteries. He reunites with Ford Prefect, who explains parallel universes and his true return, leading them on another unexpected journey.
So Long, and Thanks for All the Fish
Arthur's return to Earth is brief. He meets Fenchurch, a woman who also experienced a similar 'return' after Earth's disappearance. They fall in love while trying to understand the cosmic joke played on them. Their search leads them to discover that all the dolphins had left Earth before its destruction, leaving a message: 'So long, and thanks for all the fish.' Arthur eventually finds the real Earth, restored by the dolphins, and lives a relatively peaceful life with Fenchurch, thinking his adventures are over.
Mostly Harmless: Random Events and the Guide's Demise
Arthur Dent, now settled on a peaceful, seemingly untouched planet, is enjoying a simple life. However, his calm is broken by the arrival of his daughter, Random Frequent Flyer Dent, the result of a past encounter with Trillian. Random is a rebellious and troubled teenager. At the same time, the Hitchhiker's Guide to the Galaxy publishing company is taken over by the Vogons, who plan to replace the original Guide with a more sinister and manipulative version. This new 'Guide Mark II' can destroy all possible alternate Earths.
The Guide Mark II and the End of All Earths
The Vogons, now controlling the Guide, arrange events to bring all the main characters — Arthur, Ford, Trillian, Zaphod, and Random — to a single alternate Earth. The new Guide Mark II is a sentient, reality-altering device designed to collapse all possible Earths into a single, final destruction. Random, frustrated with her father and the universe, accidentally triggers the final disaster. All remaining versions of Earth, including the one they are on, are destroyed, ending Arthur's long, strange journey and the entire story.
Young Zaphod Plays It Safe
This short story explains Zaphod Beeblebrox's early life and his reputation. It details an event where a hyperspace bypass project threatens an endangered animal species. Zaphod, despite his usual reckless behavior, takes surprisingly careful and manipulative steps to ensure their safety. This shows a more complex and calculating side to his character, proving that even a seemingly irresponsible figure like Zaphod can act with a deeper, though self-serving, goal when needed, hinting at the reasons behind his later actions.