Young Timmy Martin and his dog, Lassie, are carried away in a balloon used by a county fair for promotional purposes. The boy's parents, Ruth and Paul, contact John Stanley of the Civil Air Patrol, which later works in conjunction with the Canadian Mounties after it becomes apparent that wind currents have carried the balloon into the wilds of Canada. When Timmy and Lassie land in a treetop, the boy lowers himself and his dog to the ground. They make their way through the dense forest, searching for food and water. They finally reach a river despite the injury sustained by Lassie in a fight with a wild boar. After camping for the night, Timmy, helped by Lassie, builds a raft, hoping to follow the river back to civilization. They soon become separated in rapids, and Chinook Pete, a deafmute Indian, finds the exhausted Timmy on the bank. Lassie makes her way to the Indian's cabin but is chased off by Pete, who wants Timmy to remain with him as a replacement for his dead son.
