An English military man and an American girl meet for the first, but not last time. The man is to be posted as a Signalman at Gibraltar (the famous rock limestone promontory at the southern tip of Spain). Gibraltar (in English hands) commanded the entrance to the Mediterranean Sea and was of great military significance.
The ports around Gibraltar were heavily mined against foreign invasion. But, a foreign power could turn the situation to advantage, if they had a man Inside the Lines of the fortification. One man could destroy a fleet of ships if he could gain access to the control room. The man an girl are united again and the fate of Gibraltar hangs in the balance