A scientific expedition is sent to Mars, but after a short time nothing is heard from them again. Years later, a military envoy arrives and reports that Mars is inhabited. It is then discovered that there is one survivor - Valentine Michael Smith. He was born on Mars, but raised by Martians after the death of his parents and the others in the first expedition. The military vessel returns to Earth with Smith, where he is hospitalized while he gains strength to deal with the higher gravity of Earth.

In the hospital he meets Gillian Boardman - a nurse who sneaks into his room (he is not supposed to have contact with women) and offers him a glass of water. Smith agrees to share water with Jill, which in Martian terms effectively marries the two. Jill shares this information with her friend and reporter Ben Caxton. Ben tells Jill that, due to his parents and a law, Smith is effectively the King of Mars as well as the majority stockholder in a billion dollar corporation. Caxton writes an article calling for Smith's release from his imprisonment in the hospital.

Ben and Jill see an interview with Smith, but Jill swears he is a fake. They realize the Man from Mars will be killed if he isn't protected. Ben gets an interview with Smith, but is the fake. He is then kidnapped by the secret police. Meanwhile, Jill finds Smith hidden in a room in the hospital. She breaks him out. They escape to Caxton's home, but the police track her down. There, Smith makes the policemen disappear. Jill takes Smith to see Jubal Harshaw, a doctor and lawyer, per Ben's original plan.

Jubal helps Smith learn English, learn about humanity, and is able to maneuver so that Smith is able to claim his inheritance without becoming a threat to the government. Effectively, he makes the Secretary General (the world leader) the executor of his finances. In this time we learn that Smith can levitate items, teleport them, and make anything (including people) disappear. It is also revealed that it is a Martian custom for water brothers (which Jubal and all those in his house now are) to eat their dead.

Smith and Jill travel together doing odd jobs to help Smith learn to grok people. They work as magicians in a carnival, among many others. Eventually Smith finally learns to laugh when he discovers it is the human way to deal with pain. He decides he wants to be a minister after they meet and become water brothers with Patty - a devout fosterite.

Smith starts a new "church" that teaches the Martian language to any who want to learn. It is set up with circles, where by the ninth circle members become water brothers. At this level, they share everything, including sexual relationships with each other. Those who learn the Martian language also learn the ability to levitate, and the other abilities Smith had. The group is eventually run out of town by the locals, and Smith allows a mob to kill him as a martyr.

The group eats Mike in order to grok him fully, but they are not upset he is dead because they know that no one can really die. They decide to continue to spread the truth that Smith taught them around the world. It concludes with a scene of Smith as an archangel.