After being separated from his mother, a newborn collie puppy roams the wilderness near a mountain lake looking for food and shelter. The puppy quickly learns to survive the many dangers inherent in nature, including the menacing threat of a mountain lion and the scenting power of a skunk. When a wolf chases him into a fast-moving river, the puppy manages to float to safety on top of a log. One day, the collie happens upon Kathie Merrick, a young girl, sunbathing on the lake shore, and runs off with her clothes. Kathie chases after the puppy and catches up to him at a watering hole. The puppy continues to run, but is stopped when two of Kathie's friends, out on a hunting expedition, accidentally shoot him. When Kathie finds the wounded puppy, she takes him to her sheep rancher friend, Harry MacBain, who dresses the dog's wounds and insures his quick recovery. Kathie tries to convince her mother to train the puppy to become a sheep herder for their ranch, but Mrs. Merrick believes that the dog is a show dog, not a work dog, and sees little value in training it. After naming the dog Bill, Kathie takes him back to Harry, who sees Bill's sheep-herding potential and agrees to train him. Time passes, and Bill grows into adulthood as an effective sheep herder and a loyal companion to Kathie. One day, during a winter snowstorm, some of the Merricks' lambs escape, and Kathie and Bill set out to find them. While walking along a dangerous precipice, Kathie stumbles and falls, but Bill pulls her back up with his teeth. The two return home with the wayward lambs, and Mrs. Merrick realizes that Bill would indeed make a fine sheep herding dog. A short time later, Bill narrowly escapes death when a truck driver accidentally hits him on a mountain road. While Bill is taken to the nearest town for help, Kathie searches for her dog in a rowboat. The boat capsizes during a windstorm, but Kathie is rescued by Harry, who had seen the boat tip over from a distance. Several days pass, and while Bill recovers at Dr. Coleman's pet hospital, Kathie loses hope of ever finding him. Coleman, unable to determine Bill's owner, sends him to the Army's War Dog Training Center, where he is renamed Duke by his new master, Sergeant Smitty, and trained to become an army dog on the combat field. When Smitty is shipped overseas to fight the Japanese in the Aleutian Islands, Bill accompanies him. Bill distinguishes himself in battle, but the sound of gunfire eventually proves too traumatic for him and he becomes shell-shocked. Smitty sends Bill home on a train, but Bill escapes into the wilderness en route. His combat experience has made Bill more aggressive, and his predatory instincts drive him to kill some chickens belonging to a rancher. The ranchers chase after Bill, who eventually makes his way back to familiar territory and into Kathie's arms. With Kathie's love, Bill eventually loses his ferocity and returns to his normal state. A court order, however, has been issued to destroy the dog, and a hearing is set to determine the dog's fate. Things look bad for Bill until his Army tattoo is discovered and he is identified as a missing war hero dog. Through Harry's impassioned pleas, the judge grants Bill's freedom and allows the dog to be returned to Kathie.