Laddie and Lassie, his son, live in England in the dog kennel of the Duke of Radling (Nigel Bruce) managed by Sam Caraclough (Donald Crisp). After World War II begins, the dog kennel is taken over by the War Department under the management of the kennel man Sam. His son, Joe Caraclough (Peter Lawford), is in love with the Duke's daughter, Priscilla (June Lockhart). Joe is sent off to pilot training and ultimately stationed at an air base during the war. In the meantime, Lassie is deemed untrainable as a war dog. Notably, he appeared gun shy and would not attack a man. Lassie then manages to run away several times to reunite 40 miles away with her owner Joe. During one such visit, he hides in Joe's airplane and is undiscovered until they are in the air during a reconnaissance flight. The plane is disabled by airborne flack and they have to parachute out with Joe holding Lassie. They are then stranded in the middle of Nazi-occupied Norway but are separated accidentally. During Lassie's attempts to track Joe, Lassie almost leads the Nazi soldiers right to Joe but both manage to escape separately. At one point, Nazis use Lassie to identify Joe from among a group of Norwegian laborers. Joe is captured but, before they return to the Nazi camp, Lassie protects Joe from being shot by Lassie attacking the Nazi soldier, and they escape again. They both sneak away on a sailboat, and return to England to be reunited with Priscilla, Lassie and his father Sam.