John Cannon risks breaking a mutual defense agreement with Victoria's father and dissolution of his marriage when his ranch hands join a posse searching for marauding bandits in Mexico.

Marshal Virgil Shanks leads the posse into Mexico and comes into strong conflict with Buck and the Cannon cowhands when he disregards the delicacy of the situation and rides boldly onto Montoya land. Manolito finds the bandit leader, El Lobo, on his father's land and attempts to move the bandits out before the posse moves in for the kill.

Mexican bandit El Lobo breaks jail and plots to plunder the Cannon Ranch supplies to avenge his imprisonment by Manolito.

When heavy rain prevents wagons from reaching the ranch with needed supplies, Buck, Billy Blue, and Manolito attempt to get a mule train through. El Lobo traps the group in an isolated canyon.

Manolito leaves the routine of the Cannon Ranch searching excitement and becomes involved with three uniformed musicians.

Mano contemplated joining El Lobo, his old bandit friend, until he learns El Lobo has robbed Capt. Ellie Strong and her two followers. He steals money from the bandit to finance Capt. Strong's trip to Tombstone to escape El Lobo and the Mexican authorities, who seek Manolito's capture for petty crimes. Buck takes an interest in Capt. Strong and joins the group. The success of the trip appears questionable when El Lobo, Apaches and Mexican authorities appear.