Robert Humphrey begins running steam-train tours in Grand Mountain which Lassie and Zoe see are scaring the park's small herd of buffalo. Graham tells Humphrey he must change the train's route. Humphrey ignores the warnings and sends the train down an old and damaged track. Now, not only are the buffalo in real danger, but every passenger on the train is at risk...and only Zoe and Lassie can prevent a disaster.