Unbeknownst to young Joe Carraclough, his beloved collie, Lassie, has been sold by his financially struggling father, Sam Carraclough, to the formidable Duke of Rudling. Set in the serene backdrop of Yorkshire, England, Joe becomes worried when Lassie fails to greet him after school, which is an everyday occurrence he deeply cherishes. The grim news is finally revealed to him when his parents, Sam and Helen, reluctantly convey the heart-wrenching truth. Poor Joe is inconsolable at the thought of losing his furry friend. Meanwhile, Lassie is confined in the duke's kennels, where she is mistreated by the harsh, Cockney dog keeper, Hynes.
Yet, Lassie proves her determination and loyalty; she cleverly digs her way out of her enclosure and appears at Joe's school right on schedule, at four o'clock, igniting a wave of joy within him. However, the reuniting moment is fleeting as his parents, despite their love for Joe and understanding of his attachment to Lassie, know they must send her back to the duke. Once more, Lassie escapes, leaping over the kennel's fence, and Joe hides with her, but they are soon discovered by his father, Sam, who insists on returning her to the duke directly.
Upon arriving at the duke's lavish estate, Joe finds a glimmer of hope in the gentle presence of Priscilla, the duke's kind-hearted granddaughter, who vows to take exceptional care of Lassie. That evening, amidst their grief, Sam imparts a lesson on honesty to Joe and explains that the duke plans to take Lassie far away to Scotland for a dog show, where she will remain indefinitely. In Scotland, Priscilla soon realizes that Hynes has been cruelly chaining Lassie within her pen and expresses her concerns to her grandfather. The duke reprimands Hynes, commanding him to take Lassie for walks around the estate. However, Hynes's ruthlessness leads to Lassie’s daring escape once again, sprinting towards the gate just as Priscilla opens it, telling her grandfather that the dog is heading back to England.
What follows is an astonishing journey where Lassie faces numerous trials and tribulations, from escaping a rainstorm to almost being killed by suspicious shepherds. After traversing through treacherous swamps and swimming across rivers, she collapses outside the cottage of Dally and Dan'l Fadden, an elderly couple who take her in. Despite their kindness, they soon realize that Lassie's heart is not content. Sensing her yearning to return home, Dally allows her to leave, letting her embark on her adventure once more.
Lassie's path eventually crosses with Rowlie Palmer, a tinker and his little dog, Toots. Although initially uncertain, Lassie gradually builds trust, joining Rowlie and Toots in various escapades. Their camaraderie is put to the test when Rowlie confronts two ruthless thieves, leading to a tragic turn of events where Toots loses his life. This heartbreak prompts Rowlie to part ways with Lassie, acknowledging her purpose-driven journey.
As she continues towards her destination, Lassie encounters challenges that test her resilience, including a near-capture by dog catchers that culminates in an injury. Despite the pain and exhaustion weighing her down, Lassie does not waver in her pursuit of returning to the Carracloughs. When she finally arrives at their cottage, the reunion is both heartbreaking and joyous, with Helen and Sam overwhelmed with emotion seeing their beloved Lassie back home, albeit in a pitiful state.
As fate would have it, shortly thereafter, the duke and Priscilla arrive at the cottage. With a newfound determination to keep Lassie, the Carraclough family hides the collie from the duke. When the duke unexpectedly offers Sam a position as his new dog keeper, it signifies a significant turning point for the family. Although Lassie's presence becomes known, the duke and Priscilla cleverly conceal their recognition.
At last, just before four o'clock—signifying their magical bond—Lassie limps towards Joe's school, culminating in a heartfelt reunion. Overcome with joy, Joe rushes to embrace his cherished collie, tears streaming down his face, reflecting years of longing. Later, the shared happiness continues as Joe and Priscilla bicycle together, jubilantly accompanied by Lassie and her new litter of playful collie puppies, embodying the enduring theme of love, loyalty, and the lengths to which friendship can go.
