Belle Starr, imprisoned for defending her crooked husband Jack against a rustling charge, is released after five years. Sheriff Hawley, convinced she was Jack's accomplice, tries to run her out of town, but Hopalong Cassidy gives her his white horse to escape to Gunsmoke Canyon, where she is reunited with her father and daughter Jackie. Meanwhile, Buck Peters, owner of the Bar 20 ranch, asks Hoppy to lead a new herd of pure-breds to high pasture, but Hoppy suggests his partner Lucky, who is lovesick for Jackie, take the job to prove himself. Lucky and ranchhand Twister successfully secure the herd until Jackie, out riding, is thrown from her horse because of a rattlesnake and Lucky has to take her home. While Lucky is away, a storm occurs and the cattle are stolen. The next day, Hawley finds Jackie's horse, and Twister reports that Lucky has run out on him and the cattle have been stolen. Hawley arrests Lucky on suspicion of being Belle's rustling accomplice, but he escapes, and Buck decides not to press charges. Unaware he is a free man, Lucky hides at the Starr ranch, and he and Jackie fall in love. Belle, who is in love with Hoppy but is sure he will never settle down, does all she can to insure Jackie's happiness with Lucky. Eventually, Belle realizes her foreman, Ringo, has been filling her range with unmarked cattle while she was in jail. Hoppy and his desert rat companion, Windy, then find a dead rattlesnake, a strand of woman's hair the color of Belle's, and Twister's corpse. The dirt on Twister's clothes leads Hoppy to an old mine shaft, which had been turned into a slaughter pit for the rustlers' stolen cattle. The mine is on Trimmer Winkler's land. Hawley arrives at the Starrs' to arrest Lucky for Twister's murder. Lucky escapes on Hoppy's horse, but is shot. When Belle, Jackie, Hoppy and Windy converge on the scene, Hawley realizes Lucky had been with Jackie the night the cattle were stolen. Jackie nurses Lucky back to health. Belle later reports that Ringo has disappeared with all her cattle, and Hoppy agrees to be her new foreman. Hoppy finds Belle's herd and marks a few head so they can trace them after Winkler buys them from Ringo. Later, when Ringo makes his deal with Winkler, Hawley finds the steer Hoppy ear-marked and places the rustlers under arrest, but Winkler shoots him. While Belle, Jackie and Hoppy shoot at the rustlers, Windy sends a signal with a mirror to the Bar 20. Before Buck's men can aid Hoppy, however, Belle is shot. When the rangers arrive, the rustlers surrender, but Belle dies. Honoring Belle's wishes, Hoppy has her buried among the rocks because "they last forever."