After the sudden death of John Carter (Taylor Kitsch), a former American Civil WarConfederate Army captain, Carter's nephew,Edgar Rice Burroughs (Daryl Sabara), attends the funeral. As per Carter's instructions, the body is put in a tomb that can be unlocked only from the inside. His attorney hands over Carter's personal journal for Burroughs to read, in the hope of finding clues explaining Carter's cause of death.
The anecdote recounts back to the Arizona Territory, where Union Colonel Powell (Bryan Cranston) arrests Carter. Powell, knowing about Carter's military background, seeks his help in fighting the Apache, insisting that Carter owes it to his country. Carter refuses, stating that he paid any debt he had when he lost his family. Carter escapes his holding cell, but is pursued by Powell and his cavalry. After a run-in with a band of Apaches, Carter and a wounded Powell are chased until they take to hiding in a cave that turns out to be the object of Carter's earlier searching, the "Spider Cave of Gold". A mysterious being, called a Thern, appears in the cave at that moment; Carter kills him but accidentally activates the Thern's powerful medallion, and is unwittingly transported to a ruined and dying planet, Barsoom (Mars). Because of his different bone density and the planet's low gravity, Carter is able to jump high and perform feats of incredible strength. He is captured by the Green Martian clan, theTharks and their Jeddak (chieftain) Tars Tarkas (Willem Dafoe). Tars instructs Sola (Samantha Morton) to watch over Carter which results in her feeding him a liquid that enables him to understand the Martian language.