The infamous submarine captain rescues six shipwreck survivors.

In the early 1860's, the submarine Nautilus I picks up six survivors shipwrecked in an Atlantic storm. The submarine takes the survivors to Templemer, a self-contained underwater city ruled by Captain Nemo, who is attempting to establish an underwater utopia. Lomax, one of those rescued, is claustrophobic and dies while frantically trying to escape. U. S. Senator Fraser tries to persuade Nemo to allow him to continue his diplomatic mission, the prevention of the spread of the American Civil War to Europe. Two brothers, Barnaby and Swallow, also wish to depart, but for a different motive: Templemer is rich with gold extracted from the sea, and the brothers plan to steal the gold and carry it to the mainland. They persuade Joab, Nemo's first mate, to allow them to commandeer the smaller Nautilus II which Nemo has based in the glass-domed city. Meanwhile, Senator Fraser and Nemo succeed in destroying Mobula, an enormous sea monster that had been threatening the city; Fraser then joins Barnaby and Swallow in an attempt to reach the surface in the stolen submarine. In the course of their escape, the ship is wrecked and Barnaby is killed, but Fraser and Swallow are rescued on the surface by a passing ship.