Hoppy and his friends Windy and Lucky are tricked into riding south of the border in answer to a fake summons. They discover that an old friend has been murdered and are aided in his capture by the killer's sister.

Hopalong Cassidy arrives at the Gonzalez ranch in Mexico with his sidekicks, Lucky Jenkins and Windy Halliday, after having received a note from Colonel Gonzalez, son of the ranchowner, asking him to meet him there. In the meantime, an escaped murderer known as "The Fox," who actually had sent the note to Hoppy as well as one to the colonel, intercepts the colonel and kills him as part of a revenge plot against the colonel and Hoppy for having imprisoned him. Hoppy realizes The Fox is linked with his friend's death and promises the colonel's father, Don Carlos, that he will capture the killer, but tells him he must lay low because he is suspicious of their houseguest, Janet Leeds, whom he believes is The Fox's sister. Hoppy's apparent indifference to finding the killer infuriates both Lucky and Don Carlos' daughter Anita, who are unaware of his suspicions. Finally, Janet leads Hoppy into the desert where he is taken captive by The Fox. It is not long, however, before Hoppy takes The Fox captive, but his plans to return to the ranch are thwarted when Lucky and Anita are shot at by The Fox's gang. Although they are outnumbered, they are able to hold the gang off while Windy, who is injured in the gunfight, returns to the ranch for help. Don Carlos and his men arrive just in time to save them. To save his own life, Hoppy is forced to shoot The Fox. Back at the ranch, Hoppy and Lucky are happy to learn that Windy will recover.