The boat draws close to the stern then turns off along one side. A man and a woman, both spies, wait for their partner to arrive. He does so and takes out a map of the plans of the submarine mines at Shoeburyness. The three pore over the map and decide on a plan to blow up the British fleet. They agree and leave. Meanwhile in the street a detective chats to one of his colleagues and notices the spies as they pass. He recognises them and follows them through a crowd to the docks. At the docks the spies look again at the map of the mines and the placing of the British fleet. Seeing the map, the detective realises the situation. The spies walk along the dunes and, as the detective passes, they set upon him and throw him down a cliff, leaving him for dead.

The detective recovers and manages to clamber up the cliff and with the help of two passers-by reaches safety and finds out where the spies have gone. The spies effect a plan to get into the naval fort to detonate the mines. The woman and one of the spies pretend a theft is occurring and the sentry rushes to her aid. The other spy sneaks into the fort. The detective arrives on the scene and with an officer rushes into the fort. Outside the mine operations room the sentry stops to blow his nose. The spy disposes of him and takes the keys and enters the room. He checks his map as the detective and officer arrive and, just as the spy is about to set off the electric mines, the detective forces his way into the room and shoots the spy dead. The officer checks the dials and the two men shake hands.

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