Early in the Second World War, U-37, a German U-boat, makes its way to Canadian waters and participates in fictional anti-shipping activities similar to those that would later characterize the Battle of the St. Lawrence (which occurred in actuality some time after the film's release). The U-boat succeeds in evading an RCAF patrol and moves north. While a raiding party of six Nazi sailors is put ashore in an attempt to obtain supplies, the U-boat is sunk in Hudson Bay. The six attempt to evade capture by travelling across Canada to the neutral United States.
Led by Lieutenants Hirth (Eric Portman) and Kuhnecke (Raymond Lovell), the small band of sailors encounter and sometimes brutalise a wide range of people, including the Eskimo Nick (Ley On), and a French-Canadian trapper (Laurence Olivier). Happening across a floatplane crew and local Eskimo onlookers, the group steals the aircraft and begins flying across Canada.
Outside Winnipeg they attempt to stir up the Hutterite community, believing them to be sympathetic to the German cause. Lieutenant Hirth's stirring speech is rejected by Peter (Anton Walbrook), the community's leader, and even by one of their own, Vogel (Niall MacGinnis), who comes to the aid of Anna (Glynis Johns), a 16-year-old girl. Vogel, who would rather join the community and ply his trade of baker, is tried by Lieutenant Hirth and summarily executed for the greater crime of trying to break away from the Nazi group.
Hijacking an innocent motorist for his car, Hirth, Lohrmann and Kranz flee west. With all of Canada searching for them, and having killed eleven civilians along the way, Lohrmann is arrested by Canadian Mounties at a public gathering. Next, Kranz confronts writer Phillip Armstrong-Scott (Leslie Howard) in a cave and despite wounding him is knocked cold.
The story comes to a head with a confrontation on a goods train between Hirth, the sole remaining fugitive and absent-without-leave Canadian soldier Andy Brock (Raymond Massey). When Hirth learns that the train has crossed into the United States at Niagara Falls, he surrenders his gun to a customs official and demands to be taken to the German embassy in the US, that was still neutral.
Brock explains that Hirth is wanted in Canada for murder but while the US border guards are sympathetic to his plea, they cannot find any official reason to send him back to Canada. Brock then points out that Hirth is locked in the freight hold but is not listed on the freight manifest. The US guards are happy to accept this pretext and send the freight car, along with Hirth and Brock, back to Canada as "improperly manifested cargo". The film ends with the train reversing to Canada and Brock about to pummel Hirth in the boxcar.
What embassy does Hirth want to go to?
German embassy