An inventor completes a novel submarine and turns it over for secret government trials, while a band of daring youths join the vessel for experimental cruises. The story follows their underwater and surface exploits against saboteurs, sea robbers, mechanical failures, thick fog, shipwrecks and an attempted mutiny, featuring torpedo scares, coded messages and a captured prize. Action scenes alternate with practical descriptions of submarine operation, and the episodes emphasize cooperation, quick thinking, and steady courage in dangerous, high-stakes maritime situations.