After public school student Roddy Berwick takes the blame for a friend’s transgression and is expelled, his life goes downhill.

Hard times come for the Carraclough family and they are forced to sell their dog, Lassie, to the rich Duke of Rudling. Lassie, however, is unwilling to remain apart from young Carraclough son Joe and sets out on a long and dangerous journey to rejoin him.