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.