The Seaview is sent to stop an active volcano from erupting and causing a catastrophe. Accompanying them is Dr. Turner who's suppose to help them. But it turns out he has a different plan and that is to make the volcano erupt so he can acquire something that will make him immortal.
A puppeteer comes on board to entertain the crew. But it is later revealed that his puppets are alive and they plan to take over the sub. That's when they replace the crew members with duplicates. Only Nelson and Crane are left to try and stop them.
Admiral Nelson and Captain Crane are shocked to see a scientist shot to death during a live televised news conference viewed from the Nelson Institute. Especially shocking is that the killer appears to be Admiral Nelson. Both Nelson and Crane are eventually able to overcome efforts to stop them from boarding the Seaview. The submarine races to stop an earth-threatening project set in motion by the radical scientist, who may not really be dead after all. In the meantime, a very dangerous master of disguise causes mayhem and confusion as he tries to stop the Seaview ...
The Seaview is sent to stop a warhead on the ocean floor from detonating. But a leprechaun wants it to go off. So he tries to stop them. But his good twin brother tries to help them stop him.