In Yorkshire, England, at the estate of the Duke of Rudling (Nigel Bruce), the British Army converted the grounds into a training camp for war dogs. The camp is placed under the supervision of Sam Carraclough (Donald Crisp), the kennel caretaker, who immediately begins the process of selecting the best dogs for training, including Laddie, the young pup of the champion collie, Lassie. Joe Carraclough (Peter Lawford), now an adult, joins the Royal Air Force (RAF) during the Second World War. Departing for training school, he is forced to leave behind his dog Lassie and her pup, Laddie.