The Restaurant at the End of the Universe is the sequel to The Hitchhiker’s Guide to the Galaxy. See that recap to get up to speed here. It is the second book in the inaccurately named five-part trilogy. This book starts off with the same characters as the first book and starts right where the previous book ends with the cast of four fleeing the planet Magrathea where Arthur Dent’s brain is wanted to decode the meaning of everything. As they leave in their ship, The Heart of Gold, they are attacked by Vogons. Under most conditions, fleeing from the Vogons would be trivial as The Heart of Gold has an improbability drive that can normally transport the ship to any part of any universe almost instantly. Unfortunately, Arthur has asked the ship’s computer to make tea with milk. This request overloads the computer’s processors and leaves the ship sitting ducks to the Vogon’s weapons. Just before they are completely annihilated, a séance results in Zaphod’s deceased great-grandfather shrinking the ship and putting it in Zaphod’s pocket while also transporting Zaphod on a journey to find the ruler of the universe.
On his journey, he finds the ruler, who lives in a shack and has no idea he is the ruler of the universe. This is an ode to the theory that only someone who does not want to be in charge is qualified to be in charge. Eventually, after the journey, Zaphod returns to The Heart of Gold and sends the ship to the nearest restaurant. Geographically, the nearest restaurant is exactly where they currently are, but many years in the future. This restaurant is built at the literal chronological “end” of the universe.
After a dinner of steaks made from a being that introduced himself prior to the meal, Zaphod decides not to leave in The Heart of Gold, but rather in another ship that is in the parking lot. It turns out that Zaphod’s depressed robot, Marvin, is also in the parking lot working as a parking attendant. Many millions of years have passed since Marvin last saw Zaphod. Marvin is, geographically, exactly where he was left when the last book ended. Zaphod has Marvin let everyone into a sleek black ship and Marvin, Trillian, Ford, Arthur, and Zaphod venture away from the restaurant.
They soon discover that the ship is, for the most part, a drone designed to drive into the nearest sun and provide a sort of pyrotechnic display for a rock concert on a neighboring planet. They discover a transporter on the ship that allows everyone to escape before colliding with the sun. Everyone, except Marvin, who stays behind to control the transporter.
The book then loses Zaphod and Trillian who are transported to a different location than Arthur and Ford. Arthur and Ford are transported to an enormous ship filled with frozen, but still living bodies, and a small crew navigators. It turns out this ship was “fleeing” its home world to populate another planet. It was supposed to be one of three ships that was travelling to a new world. It turns out that this ship was really the only one travelling. It was filled with hairdressers, middle-managers, telephone sanitizers, and others who where deemed irrelevant to their society and were tricked into being expelled as a way to rid their planet of its worthless members.
They eventually arrive at their new planet which Arthur and Ford determine to be prehistoric earth. They find a small population of inhabitants already on the planet, but soon realize they are dying off due to the new people arriving. This makes Arthur realize he is a descendant of the cast-offs on the recently arrived ship, rather than the native inhabitants. The book ends with Arthur and Ford stranded 2 million years in the past on a planet of Neanderthals and morons.