In the interests of solidifying the American presence in newly acquired California, the federal government assigns Capt. John Holmes (John Wayne) and his trusty horse, Duke, to notify settlers that the government needs them to register any land acquired under Spanish rule. Holmes and mariachi Felipe (Luis Alberni) struggle to stop the marriage of smooth-talking con artist Don Luis Gonzales (Donald Reed) to unsuspecting real-estate heiress Dolores Castanares (Ruth Hall).