Captain Anson (John Mills) is tired and thirsty. Forced to leave his North African post when it becomes clear it will be invaded, Anson is on his way to Alexandria, Egypt, and looking forward to a nice cold beer. Before he gets there, though, circumstances pressure him to take on several passengers. Sister Diana Murdoch (Sylvia Syms) and Captain van der Poel (Anthony Quayle) prove to be excellent traveling companions, but Anson soon realizes that one of them is a German spy.