In a forerunner of ''The Dirty Dozen'', Marine Gunnery Sergeant McGrath (Robert Webber) takes 12 Marines from the military prison and trains them to blow up a tunnel behind North Korean lines. McGrath's only friend on the patrol is his Korean guide Pak (Dale Ishimoto). Hating their sergeant, the Marines plan to return to their lines without him, seeing that he becomes "a casualty of war". However, en route to their target they find an injured nun (Anna Sten) and a group of Korean convent girls whose bus has been destroyed.