Señor José Valdez, a wealthy Argentinian cattle rancher, comes to the Bar 20 ranch to buy a herd of prize cattle. At the ranch, Valdez meets Hopalong Cassidy, the Bar 20 foreman, who befriends Valdez's grandson Ernesto and foreman, Fernando Rameriz. Fernando tells Hoppy about a series of tragic accidents that have befallen the Valdez family, resulting in the death of Don José's son and daughter. Consequently, when Valdez's moody son-in-law, Ralph Merritt, opposes the purchase of the Bar 20 cattle, Hoppy becomes suspicious and agrees to accompany the herd to Argentina. Hoppy's suspicions are confirmed when he learns that Don José's son died not by his own hand but was shot by a colt 44, and that both deaths happened in Merritt's absence. When Hoppy sees Merritt embracing the dance hall girl Chiquita and overhears him plotting with the killer Slim, he links Merritt to the deaths. Merritt, realizing that Hoppy is a danger to him, arranges for Chiquita to decoy him to a meeting where he plans to kill the cowboy and then raid the Valdez ranch to eliminate the old man and Ernesto, thus making himself the sole heir to the Valdez empire. Merritt's plans go awry, however, when Hoppy gets the drop on him and Chiquita discovers that he has been duplicitous in his promise of marriage. As Chiquita rides to the ranch to warn Lucky and Don José of the raid, Hoppy rounds up the loyal gauchos and organizes them into a posse. At Juanita's warning, Lucky loads Don José and Ernesto into a wagon and drives off with Merritt and his men in pursuit. Hoppy and the gauchos arrive just in time to capture Merritt and his gang, and with their job accomplished, Lucky and Hoppy return home to the Bar 20.