Lassie, poisoned by pesticides, is found by an up-and-coming race horse, Glory, and her young owner. Once well again, she befriends a newborn colt on their California thoroughbred farm--then Glory is threatened by a marauding cougar.

Even Hank can't believe all his surgical skills can heal Glory's injured leg, but Monty takes a chance and has her operated on nevertheless after a pep talk from partner Bert. Then it is up to Gary and Lassie to make her well, with exercise and swim therapy.