Life, the Universe and Everything is the third book in the Hitchhiker's Guide to the Galaxy series by Douglas Adams. The book continues the misadventures of the human and alien characters introduced in the first two novels, and follows their intergalactic journey to unravel the mysteries of the universe.

The novel begins with Arthur Dent, the last surviving human from the planet Earth, who has been stranded on the planet Prehistoric Earth. He had been rescued by a group of aliens who are part of the Krikkit war fleet, who are planning to destroy the universe. Arthur is contacted by his friend Ford Prefect, who arrives on the planet in a spaceship to save Arthur from the Krikkit robots.

Meanwhile, in another part of the galaxy, the Vogons, notorious bureaucrats and destroyers of worlds, have taken over the role of the Galactic government. They are in search of the "Heart of Gold", a ship that can travel through space using the power of improbability. However, the Heart of Gold is in the possession of Zaphod Beeblebrox, the two-headed president of the galaxy, who has surreptitiously stolen it.

As the story progresses, Arthur and Ford team up with their alien crewmates and the all-knowing supercomputer, the Hitchhiker's Guide to the Galaxy, to figure out the connection between the Heart of Gold and the Krikkit war fleet. They also discover that the Heart of Gold is powered by an ancient supercomputer called "Deep Thought", which has been tasked with answering to the ultimate question of life, the universe and everything. However, no one knows what the question actually is.

The crew travels to the planet Krikkit, where they have to prevent the Krikkit robots from destroying the entire universe. They learn that the Krikkit robots are controlled by a powerful alien race that plans to use them to take over the universe. Meanwhile, the Vogons track down the Heart of Gold and capture Zaphod along with his sidekick Marvin the depressed robot. They bring them to the planet Krikkit, and threaten to kill them if they do not surrender the Heart of Gold.

In the climactic scene, Arthur, Ford, Trillian, and the other members of their crew confront the Krikkit robots and their alien masters. They use the power of improbability to defeat the aliens and save the universe. Deep Thought finally provides the answer to the ultimate question of life, the universe, and everything, which is revealed to be 42. However, its significance remains unknown.

Life, the Universe, and Everything is a witty and satirical take on science fiction, philosophy, and the meaning of life. Adams' clever use of absurd humor and unconventional storytelling make the novel an entertaining and thought-provoking read. The characters are quirky and endearing, and their interactions add humor and depth to the story. The book's exploration of complex philosophical questions, such as the purpose of existence and the nature of reality, give it a deeper meaning beyond its comedic elements.

Overall, Life, the Universe, and Everything is a delightful and engaging science fiction novel that continues to captivate readers around the world. Its unique blend of humor and thought-provoking themes make it a timeless classic of the genre.