In 1942, amiable RAF reconnaissance pilot Peter Ross (Alec Guinness) is on his way to Egypt when he is stranded en-route to Egypt on the besieged island of Malta and gets caught up in the effort to save the island from the Luftwaffe. On studying aerial photographs he discovers that an imminent invasion of Malta is being prepared. It is of vital importance that the island remains under the control of the Allies, and Peter is the man selected to trace and destroy Rommels supply lines. He begins to plot the enemy's Mediterranean convoy movements - but he is distracted by the thought of romance with a local girl, Maria (Muriel Pavlow).