Roger accidentally sets off an old World War II mine in the lagoon. The explosion irritates a mutant pre-historic creature, which Tarzan had previously heard of from the local natives. Jane doesn't believe Roger really saw the legendary creature, until it kidnaps her, and imprisons her in an underwater cave. Now, Tarzan must save her, despite the newly floating, explosive mines. Even if Tarzan captures the creature, will the benevolent ape man give the creature its final punishment?