In Wyoming, wannabe outlaw Butch Cassidy joins forces with sharpshooter Harry Longabaugh (who renames himself the Sundance Kid) and they carry out a series of robberies. However, Butch is stalked by a member of his former gang, O.C. Hanks, who believes he betrayed the gang. The two spend the winter in Telluride until they hear O.C. is in town, then rush away to deliver diphtheria serum to snowbound farms and become heroes. O.C. ambushes them and wounds Sundance by mistake. He recovers in Butch's home, tended by Butch's wife Mary and their two sons, who don't know their father's real job (he takes up butchery to earn money). O.C. turns up for a showdown and Sundance unintentionally kills him. Returning to crime, the pair rob a bank and then decide to rob a money train guarded by cavalry, not knowing Butch has been promised an amnesty if he gives up crime. They get the money, free the cavalry horses to prevent pursuit, and ride away dreaming of being famous outlaws.