This book takes the concept of eminent domain to a marvelous extreme. Arthur Dent, a mild-mannered Englishman faces the imminent destruction of his house. His house is in the path of a proposed highway. Mr. Dent valiantly attempts to thwart the process. Ford Prefect, a friend of Arthur's, comes upon the scene, claiming that the loss of a house in England is not the real crisis. The actual problem seems to be the imminent destruction of Earth for an intergalactic bypass. Ford reveals that he actually is an alien from a planet near Betelgeuse. Instead of being an actor as claimed, he is a writer for the Hitchhiker's Guide to the Galaxy. Arthur is not pleased to learn that Ford has labeled Earth as "mostly harmless." After grabbing their towels, an essential travel aid according to Ford, the two hitch a ride on a Vogon spaceship seconds before Earth goes boom. After being subjected to bad Vogon poetry and being ejected from the spacecraft, Ford and Arthur hook up with Ford's relatively strange relative, Zaphod Beeblebrox, Zaphod's girlfriend, Trillian (once Tricia McMillan of Earth) and Marvin, a very depressed mechanoid. Eventually, the friends arrive at the planet Magrathea, once the center of the planet building or terraforming industry.
There, they meet Slartibartfast, an architect who was the designer of Norway's fjords. The architect tells them the tale of the computer, Deep Thought that was given the task of providing the answer to everything. After an enormous amount of time, Deep Thought came up with the answer, 42. As it is on occasion, this answer forced the realization that it made no sense and that the actual question was unknown. So, a computer designed to uncover the question was built. Ironically, the computer was Earth. Timing is crucial; therefore, just before the computer revealed its answer, Earth was destroyed. Arthur learns that the builders of Deep Thought are mice. Unfortunately for Arthur, the mice believe that he, being from the computer/planet, subconsciously knows the ultimate question. Surgery by rodent is scheduled. Arthur avoids an operation by leaving with his friends. They head for the "Restaurant at the End of the Universe" for a meal and a view of the Big Bang.