In 1881, Edgar Rice Burroughs attends the funeral of his uncle, John Carter, a former American Civil War Confederate Army captain, who died suddenly. The body is put in a tomb unlockable only from the inside. The attorney executing the will hands Burroughs Carter’s journal, which he reads, hoping to find clues about Carter’s death and the reason he is the willed heir. The bulk of the film enacts what Burroughs reads.
While prospecting in the Arizona Territory in 1868, just after the Civil War, Carter is arrested by Union Colonel Powell, who seeks his help in fighting the Apache and insists that Carter owes it to his country. Carter refuses, stating that any debt was paid when he lost his family. Carter escapes from his cell but is pursued by Powell and his cavalry. After encountering a band of Apaches, Carter and the wounded Powell take refuge in a cave, which turns out to be the object of Carter’s earlier search: the “Spider Cave of Gold”. A mysterious priestly being, a Thern, appears in the cave at that moment, and surprised by Carter’s presence, immediately attacks him with a knife; Carter kills him but accidentally activates the Thern’s powerful medallion, and is unwittingly transported to the ruined and dying planet of Barsoom, later revealed to be Mars.
Due to Barsoom’s low gravity and his own relatively high bone density, Carter is able to perform incredible jumps and feats of strength. He is captured by the Tharks, a clan of four-armed Green Martians led by their “Jeddak” (chieftain), Tars Tarkas. Tars instructs Sola to watch over Carter, leading to her feeding him a liquid that enables him to understand the Martians’ language.
Elsewhere on Barsoom, the human Red Martian city of Helium, led by Thardos Mors, and the Walking City of Zodanga, led by the villainous Sab Than (Dominic West), have been at war for a thousand years. Sab Than, who wants to conquer all Barsoom, is armed with a special and highly destructive weapon given him by three Therns in the midst of a ship-to-ship close quarters combat, which the weapon ends by annihilating all the other participants. Sab Than proposes a cease-fire with Helium and an end to the war, sealed by marriage to Mors’ daughter, the Princess of Helium Dejah Thoris. The unwilling Princess, disguised as a soldier, escapes in a Helium ship.
As Tars Tarkas tries to get John Carter to show off his jumping abilities, a Thark sentry spots a ship from Helium and another from Zodanga, and the Tharks scatter to hide. Carter races into action and leaps to save the human Dejah from falling. He kills some Zodangan soldiers and briefly fights with Sab Than. Than’s ship retreats and John Carter is hailed by Tars Tarkas as “Dotar Sojat” (roughly translatable as “My Right Arms”), for his strength and skill. Tarkas even grants him Dejah as part of the Thark spoils. Later, Carter, accompanied by Dejah, tries to find a way to get back to Earth, and stumbles upon a temple ruin sacred to the Tharks; Sola encounters them there and vainly tries to stop them from entering. They discover an inscription depicting a way to Earth in the sacred river of Iss, before Carter, Dejah, and Sola are caught by Sarkoja and Tal Hajus. The three are sentenced to death according to the Thark code, but are helped to escape by Tars Tarkas, who reveals to Carter that Sola is his daughter. Tal Hajus and Sarkoja find the prisoners gone, and the ambitious Hajus declares that Tarkas has betrayed them.