The narrative follows Bob Steele, a skilled young submariner who is entrusted with testing the Grampus on a record deep dive and later with executing sealed orders that propel the vessel into political and maritime danger. The crew faces mechanical hazards, underwater combat with a gigantic bull cachalot, a mutiny and treachery among allies, and a series of daring missions including running a battery, capturing a military leader, clandestine searches for documents, and a hazardous voyage up the Amazon. Episodes alternate technical submarine practice, hand-to-hand confrontations, boarding actions, and suspenseful escapes as loyalties shift and risks escalate.