Plot

An Earl's cadet son stops a German baron from planting wireless-controlled signals at Brighton.

In the years before the Great War, Lord Billy is the young son of the widowed Lord Halsford. The motherless child lives a lonely life in his castle, as his kindly father is a busy politician. His only friend is his dog, and he spends long days travelling in his imagination to the days when his ancestors defeated the Viking hordes.

As part of a government initiative, Lord Halsford is asked to host Baron Hausen, and a German delegation arrives at the castle. Billy surprises the Germans surveying the surrounding countryside, and unsuspecting, is helpful and friendly. However, he is treated with rudeness and hostility by the Baron, and retaliates, resulting in an unjust punishment.

Meanwhile, three cadets have spotted German reconnaissance operations as they photograph undefended stretches of coast in preparation for invasion. When the delegation leaves, Lord Billy extracts his revenge by knocking the Baron's top hat off with his catapult, and teams up with the cadets, who promise to show him how soldiers fight.

In the cadets, Billy shows himself to be officer material and to embody his ancestral courage by defending a smaller boy from a bully. He settles his differences with the bully in the boxing ring - the cadets teach boys to be men - and defeats him, much to his father's delight.

War breaks out and Baron Hausen is now a crack aviator, planning to attack an undefended town on the English coast by means of a wireless signal operated from a U-Boat. To do this, he intends to land a small force and deploy transmitters along the stretch of coast he previously surveyed. Only the cadets, under newly-promoted Lance Corporal Lord Billy stand in their way.

The cadets witness the Baron landing in a captured English plane and keep the enemy under observation as they proceed with their treacherous plan. A nervous German accidentally sets off the signal lights, allowing Billy to recognise his old enemy, the Baron. The cadets fix bayonets and charge the plane, knocking out the sentry while the rest of the Germans are testing their equipment. Under Billy's instructions, the boys steal the plane and capture the Germans.

Later, Billy's father watches him receive a medal for his courage from the Prince of Wales.