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In 1941, a German submarine drops off some aviators near Hudson Bay in Canada.  They meet two other men with dog teams and sleds and travel north.  One day, an avalanche buries everyone except for Hugo von Keller.  Nearly dead with exhaustion, Hugo is found by two members of the North West Mounted Police, Steve Wagner and Jim Austin.  When Hugo learns that Steve is of German descent, he questions him closely about his feelings for Germany.  Steve says only that now he is a Canadian, but arranges matters so that Jim goes for the authorities, leaving him alone with Hugo in the Mounties' isolated cabin.

In town, before he reports to the commander, Jim tells Laura McBain, Steve's fiancée, about their prisoner, and she assumes that Steve will earn a promotion that will allow them to be married.  Inspector Barnett, Steve and Jim's superior officer, on the other hand, professes to be worried about Steve's loyalty and orders the prisoner to be brought in.  While alone with Hugo, Steve has made an attempt to discover why the German is in Canada, but his questioning ends when the other Mounties arrive.

Hugo is sent to an internment camp, but he soon escapes with several other men.  Steve is called back for questioning and, as part of a plot to discover the Nazi objective, is discharged from the force.  As he leaves his hearing, Steve angrily curses Canada and is taken to jail.  Later, he is bailed out by a man named Ernst Willis, who offers him a job guiding him to the north country.

On the train north, Steve secretly meets a Mountie and reveals his itinerary.  Willis is unsure where Steve's loyalties really lie, however, and, despite Steve's attempts to cover for the man, shoots him.  Steve and Willis then jump off the train and rendezvous with the escaped Germans at an Indian cabin.  At first Hugo pretends to believe that Steve is no longer loyal to his country, because Steve is the only one who can guide him through the Canadian wilderness.

To make sure that Steve will not betray him, however, Hugo abducts Laura, and holds her hostage.  Steve is required to help three more German POW's escape and takes the opportunity to send an encoded message to headquarters.  Jim then follows their trail, but is spotted by Hugo and killed.  After Willis falls ill, Hugo kills him as well.  Steve then convinces their Indian guide to go for help, but he, too, is killed.  The men finally reach their destination—a mine shaft where bomber parts have been secreted.  When the plane is assembled, Hugo plans to use it to bomb the Panama Canal.  Steve is forced to help with the assembly, but at the last minute, he overcomes his guards and boards the plane.  There, he shoots the entire crew and parachutes out before the plane crashes.  Later, he and Laura marry.