John is a cheerful, energetic boy living in a small English coal-mining town. His life transforms when he discovers a lost puppy among a flock of sheep and brings her home, naming her Lassie. The two forge a deep, sibling-like bond, and Lassie wins the entire community's affection through her intelligence and loving nature.

The town faces crisis when the coal mine owner declares closure due to depleted resources. Demonstrating initiative, John directly approaches the owner to request further investigation for remaining coal. The owner agrees but imposes a harsh condition: Lassie must be given up and sent to Scotland.

Enduring Lassie's absence, John actively participates in community efforts. He defends his father, Sam Carraclough, during a mine conflict and supports initiatives to revitalize the local economy, as seen in events like "John's Decision and Mine!" and "Do Your Best to Defend John, Lassie!". His friend Priscilla, the mine owner's granddaughter, aids him by freeing Lassie from confinement, enabling the dog's arduous journey home.

The narrative concludes with Lassie successfully returning. John reunites with her, and the pair symbolically run towards future aspirations together in the final episode.