After capturing a German boat off the coast of South America during World War I, two officers in the British navy battle the enemy aboard a convoy cruiser, merchant vessel, and pirate ship.

During World War I, two young officers in the British navy battle the Germans in the Adriatic Sea, Indian and Atlantic Oceans, and Persian Gulf aboard a battle ship, torpedo submarine, and hydroplane.

"One!" The speaker was Jack Templeton, an English youth and first officer of the British submarine D-16, Lord Hastings, commander. Frank Chadwick, an American lad about the same age as Jack, the second officer of the under-sea fighter, laid his hand upon his friend's arm. "Let me take a look," he said. Jack relinquished to his chum his place at the periscope, and the latter peered into the instrument long and earnestly. Into the periscope-which protruded slightly above the surface of the water while the submarine was still submerged-came the vision of a sinking warship, and the sight, enlarged by powerful binoculars, was apparent to Frank's eyes. "She's done for, all right," he said quietly, turning away at length. "Pretty good shooting, I should say."

As World War I draws to a close, two young American officers engage in a series of missions which deal crippling blows to the German enemy.

Hal and Chester undertake a number of missions in Europe just after the Americans enter World War I.