Swendler (Denis Goacher) assures Mark (Gerald Flood) that Professor Ziebrecken (Aubrey Morris) has no intention of using the nuclear missiles, but they are for defense only. Mark is not quite assuaged by Swendler’s reassurances. When they reach the surface Swendler tells Ziebrecken that Mark knows about the warheads. Ziebrecken is unaffected by this news. Swendler is a little bothered about them. Ziebrecken assures Swendler that he has no intention of attacking anyone. He wants peace and a world united with him as its leader. Ziebrecken fully believes that the world will surrender to him with just the threat of using the missiles.
Mark tells Peter (Stewart Guidotti) about the warheads. They discuss some way of letting the outside world know where they are, but Professor Westfield’s (Hayden Jones) transmitter has been taken apart to be modified. Mark and Swendler need to go back down to the cellar and retrieve the remaining warheads. As they are returning Peter gets the idea to throw an air cylinder over the guyot and into the current. The current carries the cylinder down to a depth where it implodes. The implosion sends a shock wave that cripples the bathyscaphe with Mark and Swendler aboard.
Hoping that something can be done to save Mark, Peter confesses to Ziebrecken what he did. On board the bathyscaphe Mark and Swendler manage to lighten the vessel to make it lighter than the surrounding water. They float back up to the surface. The implosion is heard by a ship, but they were unable to get an exact bearing. Whether or not it means anything is difficult for the Royal Navy’s Captain Payne (Peter Williams) say.
In the meantime, Professor Ziebrecken has had enough with Mark and Peter trying to sabotage Aegiria. He orders that Peter be executed. Swendler suggests that Peter can still be useful with testing the improvements that Professor Westfield did on the communication system. Ziebrecken agrees. He tells Swendler to take Mark and Peter with him in the Cyana to deliver the warheads to the underwater missile sites and test the radio. He also tells Swendler to make sure that Mark and Peter have a fatal accident on the way back.
Spanning only one season “City Beneath the Sea” consisted of seven, twenty-five-minute episodes. It was produced by ABC, Associated British Cinemas. It was one of many television companies that were established by cinema chain companies.