Out of the Silent Planet is a science fiction tale of Dr. Elwin Ransom's journey from Earth to live among the inhabitants of Mars, called Malacandra in the local language. Through the medium of science fiction, the novel addresses several social issues such as racism, colonialism and religion. It is the first book in C.S. Lewis' Space Trilogy.

While on a solitary walking tour of the English countryside, Dr. Elwin Ransom, a college language professor, is kidnapped and drugged by two men. He awakens to find himself aboard a spherical spacecraft. His captors are Devine, a wealthy and greedy Englishman with a sinister streak, and Weston, the brilliant physicist who designed the spacecraft. Weston explains they are bound for one of the other planets in the solar system, but does not reveal which, except to say that the inhabitants of the planet call it Malacandra.

Ransom learns that Weston and Devine intend to hand him over to the inhabitants of Malacandra, called sorn. He imagines the sorn to be terrible creatures intent on killing or sacrificing him and decides he would prefer suicide to such a death. He resolves to escape or to die trying.

Ransom does manage to escape his captors. Just as they are about to hand him over to a group of sorn, they are attacked by a dangerous sea creature called a hnakra. In the confusion of the attack, Ransom escapes into a forest of enormous plants.

Ransom wanders in the forest for a time, and comes across a seal-like creature which seems to be talking to itself. Deciding the creature must be intelligent, Ransom makes signs to it, managing to communicate that he is hungry. The creature, which Ransom learns is called a hross, feeds him and takes him by boat to a canyon where other hross live.

Ransom lives among the hross for several weeks and gradually learns the language and culture of the people of Malacandra. The hross, he observes, live in the canyons, use simple tools and enjoy poetry and song. The sorn, he is told, inhabit the higher areas and are knowledgeable in history, astronomy and science. The pfifltriggi are a third race, skilled at making beautiful and useful objects. Ransom repeatedly asks which race is dominant, or in charge of the planet. The hrossa seem confused at this notion, and answer that Oyarsa, an entity that lives in a place called Meldilorn, has reign over the entire planet.

Ransom is summoned to Meldilorn by an eldil, the nearly invisible creatures who serve Oyarsa. Assisted by the hross and a sorn named Augray, Ransom makes his way to Meldilorn, which is inhabited by all types of Malacandrians. While examining an artistic representation of the solar system in Meldilorn, Ransom realizes that Malacandra is Mars.

Like the eldil, Oyarsa is invisible but can be heard. Weston and Devine have also been summoned by Oyarsa, and the three men are interrogated. Ransom learns that Devine's intentions are to take gold from Malacandra back to Earth and that Weston intends to pave the way for humans to take over the planet. Oyarsa quizzes Ransom for a long time about the Earth, revealing to him that a creature similar to Oyarsa once had rule over the Earth, but had not been heard from for hundreds of years. For that reason, Earth is called Thulcandra, which means "silent planet."

Oyarsa tells Weston and Devine they must return to Earth in their spaceship but invites Ransom to stay on Malacandra if he wishes. Ransom decides to return with the two men.

The next to last chapter of the book is written from the viewpoint of the narrator and explains how he came to learn of Ransom's adventure and to write a fictionalized account of it. Finally, a fictional excerpt from a letter by Ransom himself is presented. These chapters link the events and characters presented in the novel to current events, and foreshadow a possible great upheaval among the planets of the solar system.