AllMovie

While on the Seaview's glass-nosed observation deck, Captain Crane (David Hedison) spots a mermaid (Diane Webber) swimming ahead of the ship. No one else believes him, but he is unable to lose his fixation on this creature, and eventually goes out on his own to search for her. After finding the mermaid and bringing her aboard, Crane suddenly finds himself faced with two additional pressing tasks -- a deadly sea monster that comes aboard, seemingly searching for the mermaid; and a mission to track down and disarm a hidden super-powered nuclear weapon hidden in nearby waters.