Picking up where the "Poseidon Adventure" left off, this action packed yarn tells the story of how a group of rescue workers descend on the overturned Poseidon to try and salvage the remnants of the crippled ship. Among them, posing as a medic, is the nefarious Captain Svevo, who arrives in order to steal a nuclear load the liner was carrying. During a struggle, Svevo and the team of rescuers become trapped in the overturned hull. When they try to find an exit to the surface, they discover a handful of survivors from the original disaster. Now they must put their differences aside and join forces to find their way out of this watery grave.