During World War II, a dog kennel on the Yorkshire, England estate of the Duke of Rudling is commissioned by the British army to be converted into a training camp for war dogs. The camp is placed under the supervision of Sam Carraclough, the kennel caretaker, who immediately begins the process of selecting the best dogs for training. When Laddie, the young pup of champion collie Lassie, is picked for training, Sam's son Joe, a Royal Air Force cadet who is visiting the estate on furlough, tells his father that Laddie is not qualified for the job. Joe's predictions that Laddie is too obstinate and skittish for the job soon prove true when Laddie resists the training and fails all tests of courage. One day, when Joe leaves the duke's estate to rejoin his regiment, Laddie chases after his bus and follows him to his camp forty miles away. Joe tries to hide Laddie in his bed, but the dog is discovered by the warrant officer and ordered removed. Sam takes Laddie back to the duke's estate, but Laddie escapes again and tries to join Joe on a reconnaissance flight. Sgt. Eddie Brown, Joe's army pal, restrains Laddie, but the dog breaks free and chases after the plane as it races down the runway. Laddie is eventually recaptured and held at the camp until Joe returns. When Joe is sent on his next mission, a dangerous reconnaissance flight to investigate enemy troop movements in Norway, Laddie secretly follows him into the cockpit of his plane and hides there. Joe does not realize that Laddie is on the plane until it is airborne. Somewhere over Norway, the plane comes under enemy fire, and Joe, with Laddie in his arms, is forced to parachute to safety. Joe makes a hard landing in the remote countryside and is knocked unconscious. Laddie tries to help Joe by summoning two Nazi soldiers at a nearby lookout post, but by the time Laddie returns with the soldiers, Joe is gone. The soldiers suspect that Laddie belongs to an English soldier, but when they try to read his collar to identify his owner, Laddie flees. One of the soldiers fires his gun at Laddie, but Laddie manages to escape with only an injured paw. After some young Norwegian children find Laddie and tend to his wounds, Laddie runs off into the wilderness to find his master. Joe, meanwhile, is found by the Nazis and placed in a prisoner-of-war camp. He soon manages to escape, however, and takes refuge with Anton, a Norwegian fisherman, and his wife. Laddie, who has picked up his master's scent and followed it to the prison camp, is captured there by Nazi guards, who decide to use him to help recover their escaped prisoner. Laddie leads the soldiers directly to Joe, who ignores Anton's suggestion that he strike his dog to show the Nazis that it is not his. Joe instead gives himself away by returning his dog's affection, and this results in his return to prison. Laddie comes to the rescue, however, when he attacks the Nazi guard escorting Joe to prison. After knocking the guard unconscious, Joe eludes the Nazis and, with Laddie, escapes back to the fishing village, where he commandeers a fishing boat and returns to England.