Lassie befriends a diabetic poodle named Sparky who stows away on the Looking Glass, where there's no insulin, to be with his owner.

Hoping to help his self-esteem, Garth takes Jimmy back to the ranch with him. There the uncertain boy meets Lucy Baker, a girl who's been deaf most of her life, and her pet wolf Mountie. But while Jimmy and Lucy get to know each other, Mountie, playing with Lassie, is shot by a shepherd who doesn't know he's a pet.

After Lucy and Jimmy find Mountie in the cave where Lassie has hidden him, Lucy sends Jimmy for help on her horse. He is frightened to do so, but does so for her sake, and despite being thrown, finds Domenic, the shepherd who shot Mounty. Meanwhile, Lassie returns to the ranch.