
Stony Brooke

Tucson Smith

Lullaby Joslin
Doris Moore

Hackett

Captain Gardner

Bobby

Orphanage Assistant Mary

Unable to legally capture and sell a herd of protected wild horses, corrupt rancher Rance Macgowan uses his trained killer horse, Volcano, to substitute for the real leader of the herd and cause havoc and death among the ranches. With the government about to drop the restrictions on rounding up the herd, the Three Mesquiteers find themselves in the middle of the controversy after their friend, Sheriff Miller is killed by Volcano.

The Cattlemen's Association has called in the Mesquiteers to find cattle rustlers. They get Tex Riley to pose as Stony so Stony can arrive posing as a wanted outlaw. This gets Stony into the gang of rustlers and he alerts Tucson and Lullaby as to the next raid. But Hartley is on hand and unknown to anyone is the rustler's boss and he joins the posse with a plan that will do away with the Mesquiteers.

In this western, the Three Mesquiteers must find a killer and his band after they murder an official from the State Agricultural Service who had come to investigate an outbreak of hoof and mouth disease. The killer is fearful that the official would quarantine his entire herd. Unfortunately for the foolish rancher, if the herd is not isolated, all of his cows and those of his neighbors will die anyway. The heroes are assisted by Buck the clever Great Dane.