Canadian Commando Major Jamie Wilson (Lloyd Bridges), plans an audacious Combined Operations raid on the Axis held French port of Le Clare; if destroyed, the Germans would be stripped of the only dry dock capable of servicing their large battleships. Wilson's plan, code named "Operation Mad Dog", is to ram a destroyer packed with tons of explosives into the outer gate of the dock, while his commandos cause havoc to the dock facilities and garrison, and then detonate the explosive laden destroyer. Opposed to Wilson is Royal Navy Captain Owen Franklin (Andrew Keir), whose own son was killed on Wilson's disastrous previous Dieppe-type raid on the French coast at Le Plagé.