Prior to World War III, the crewed spacecraft Envoy is launched toward Mars, but all contact is lost shortly before landing. Twenty-five years later, the spacecraft Champion makes contact with the inhabitants of Mars and finds a single survivor, Valentine Michael Smith. Born on the Envoy, he was raised entirely by the Martians. He is ordered by them to accompany the returning expedition.
Smith is confined at Bethesda Hospital, where, having never seen a human female, he is attended by male staff only. Seeing that restriction as a challenge, Nurse Gillian Boardman eludes the guards and goes in to see Smith. By sharing a glass of water with him, she inadvertently becomes his first "water brother", a profound relationship by Martian standards, as water on Mars is extremely scarce.
Gillian's lover, reporter Ben Caxton, discovers that Smith is extremely wealthy. Ben is seized by the government, and Gillian persuades Smith to leave the hospital with her. Gillian takes Smith to Jubal Harshaw, a famous author, physician and lawyer. Eventually, Harshaw arranges freedom for Smith and recognition that human law, which would have granted ownership of Mars to Smith, has no applicability to a planet that is already inhabited by intelligent life.
Smith becomes a celebrity and is feted by the Earth's elite. He investigates many religions, including the Fosterite Church of the New Revelation, a populist megachurch in which sexuality, gambling, alcohol consumption, and similar activities are allowed and even encouraged and considered "sinning" only when they are not under church auspices. Smith has a brief career as a magician in a carnival, in which he and Gillian befriend the show's tattooed lady.
Smith starts a Martian-influenced "Church of All Worlds", combining elements of the Fosterite cult with Western esotericism. The church is besieged by Fosterites for practicing "blasphemy", and the church building is destroyed, but unknown to the public, Smith's followers teleport to safety.
Smith is arrested by the police, but escapes and returns to his followers, later explaining to Jubal that his gigantic fortune has been bequeathed to the church. With that wealth and their new abilities, church members will be able to reorganize human societies and cultures.
Smith is killed by a mob raised against him by the Fosterites. Jubal and some of the church members return to Jubal's home to regroup and prepare to found new Church of All Worlds congregations. Smith reappears in the afterlife to replace the Fosterites' founder, amid hints that Smith was an incarnation of the Archangel Michael.
