A faithful collie undertakes an arduous journey to return to her lost family.

Unaware that his poor, unemployed father Sam has been forced to sell his beloved collie Lassie to the Duke of Rudling, young Joe Carraclough, of Yorkshire, England, is immediately concerned when the dog fails to meet him, as usual, after school. When Sam and his wife Helen finally break the news, Joe is inconsolable. Lassie, meanwhile, is taken to the duke's kennels, where she is locked in a pen by Hynes, the cruel, Cockney dogkeeper. The next day, however, Lassie digs her way out of her pen and shows up at Joe's school at the usual time, four o'clock. Although Joe is overjoyed to see Lassie, his parents know that they must return her to the duke and reluctantly hand her over to Hynes. Lassie soon escapes a second time by jumping over the pen's fence. This time Joe runs off and hides with Lassie, but the two are quickly found by Sam, who insists that Joe return Lassie to the duke in person. At the duke's estate, Joe is somewhat cheered by the presence of Priscilla, the duke's sympathetic young granddaughter, who promises to give Lassie special care. That evening, Sam lectures Joe on the importance of honesty and informs his son that the duke is taking Lassie hundreds of miles away to Scotland for a dog show and will be staying there indefinitely. Later, in Scotland, Priscilla notices that Hynes has chained Lassie inside her pen and complains to her grandfather. The duke soundly chastises Hynes and orders him to walk Lassie around the estate's grounds. Hynes is so rough with Lassie, however, that the dog breaks away from him and dashes to the front gate, where Priscilla and the duke are standing. Priscilla opens the gate and allows Lassie to escape, then comments to her astounded grandfather that the collie is headed south, toward England. Lassie runs and runs until a rain storm forces her to take shelter. The next day, she resumes her journey and is almost killed by two shepherds, who suspect her of killing their sheep. After traversing a swampy area, Lassie then swims a river into England. Once in England, an exhausted, hungry Lassie collapses outside a cottage owned by Dally and Dan'l Fadden. Although the elderly couple eagerly adopt the dog, Dally soon realizes that Lassie, who whines to be let outside every afternoon at four, is not happy. Sensing that Lassie is anxious to continue her trek but is too "polite" just to go, Dally orders Lassie to leave the next day at four. Lassie takes off and eventually meets up with Rowlie Palmer, a tinker who travels with his little dog Toots. Lassie is cautious around Rowlie, but accepts his food and follows his wagon. Later, as Rowlie is selling his wares in a village, Lassie performs with Toots, doing tricks that Rowlie has taught her. That night at his camp, Rowlie is attacked by two roving thieves, Buckles and Snickers. Both Toots and Lassie help Rowlie in the fight, and when Buckles mortally wounds Toots, Lassie unleashes all her fury on him. Although the crooks scamper away, Rowlie is crushed by Toots's death. Soon after, Rowlie parts with Lassie, sensing as the Faddens did, that she is on a special journey. Lassie then makes her way through a city and is pursued by two dog-catchers. To avoid capture, Lassie jumps from a warehouse window, injuring her leg. Despite exhaustion, hunger and lameness, Lassie perseveres until she arrives at the Carracloughs' cottage. Helen and the still out-of-work Sam are stunned to see Lassie, filthy and thin, but gladly welcome her home. Soon after, the duke and Priscilla drive up to the cottage, and Sam and Helen, now determined to keep Lassie, hide her. To their surprise, the duke offers Sam a job as his new dogkeeper, and Sam gratefully accepts. Lassie soon gives her presence away, but the duke and Priscilla pretend not to recognize her. Then, just before four o'clock, Lassie limps over to Joe's school for her long-awaited reunion. Upon seeing Lassie, Joe, who has never stopped yearning for his dog, runs to her side and gives her a joyous, tearful embrace. Sometime later, Joe and Priscilla enjoy a bike ride together, accompanied by Lassie and her sprawling litter of collie puppies.