When a missile is destroyed by apparent ultrasonic sound, the Seaview investigates with Adams, the mentally unstable launch commander, on board and finds an giant manta ray source which along with Adams' actions pose a threat to safety.

Nelson is to meet a scientist working on a secret project at Loch Ness. When he arrives he discovers the scientist is dead. A series of strange events leads Nelson to believe there may be a way for a submarine to enter the Loch.