The Seaview is sent to investigate the wreck of the submarine Angler. They uncover evidence of strange behavior minutes before the loss, and Nelson sees this change in behavior as the secret to unlocking what happened to the Angler.

This episode involves the testing of a new air mixture that will allow a submarine to go to the "bottom of the sea" literally! A brash, news hogging, admiral and his aide, along with his assistant all successfully make the first tests in a diving bell. Then it is time for the Seaview to follow suit. Only then does the real fun begin!