The Seaview is on a joint mission with Russian scientists to salvage the remains of a Soviet undersea lab. While a scientist is being lowered in the diving bell, a giant whale arrives and rams the bell. The glass is shattered, killing him. Nelson and the remaining Russian scientist, Katya Markova, go on a second dive. The whale reappears, this time swallowing the diving bell whole! Now it's a race against thime as Captain Crane and the crew of the Seaview find the diving bell and get it out before Nelson and Markova run out of oxygen!

A message is received from a US sub presumed sunk in the last months of WW2. The Seaview is sent to try and locate a cave that is mentioned in the message and to try and rescue the 5 men believed to be still alive.