When brutish Joe Gilmore forces his attentions on his father's employee, bookkeeper Mary Carson, Captain, a police dog, attacks him in her defense. Out of spite, Joe decides to "take care" of Captain and his pregnant mate Lady by muzzling them and leaving them to die in the nearby California desert. Soon after, local residents discover that their chickens are being stolen by a wild dog and, during a town meeting, demand that the perpetrator be caught and killed. A posse is formed to apprehend the dog, and a twenty-five dollar reward is offered for his capture. While the posse is scouring the countryside, John Z. Blake, an Eastern-born lawyer who is romancing Mary, falls into a creek while fishing and is knocked unconscious. John is saved from drowning by Captain, but when he tries to pet the dog, Captain runs away and is caught by Joe, who is jealous of John's relationship with Mary. Although the posse is convinced that Captain is the thief, John demands that the dog receive a fair trial. Fearful of John's threats of lawsuits, the townspeople agree to give Captain a trial, and Judge Simmons is brought in to hear the case. The judge and jury are unimpressed by John's legal arguments and are unmoved by his story of Captain's heroics. Before they can claim Captain, however, the dog bolts from the court with John's help. John and his friend, Reb, retrieve Captain at the creek and also discover the still muzzled, starving Lady and her brood of newborn puppies. Back in court, John attempts to justify Captain's thefts by comparing him to a husband who is desperately trying to save the lives of his starving wife and children. Although the jurors and judge fall in love with the puppies, they still insist that Captain is a dangerous killer. At that moment, however, Mary enters the court and demands that Joe be put on the witness stand. Although Joe denies John's accusations that he abused the dogs and left them to die in an inhumane fashion, Mary demonstrates how Captain attacked Joe for getting too familiar with her and thus establishes that Joe had a motive for wanting the dogs dead. Captain is then vindicated, and John and Mary continue their romance.