Siren II, a submarine under the command of Capt. Phillips (R. Lee Ermey), ventures into the deep on a mission to recover the missing Siren I. Their quest is hindered when the submarine becomes entangled in a patch of toxic seaweed. Nearby, Phillips and his crew find an air-filled underwater cave where the government conducts sinister genetic experiments. The heroic rescue mission soon transforms into a desperate fight for survival as the crew battles hordes of mutant sea creatures.