When Bannerman returns to his office, he is knocked unconscious by Sanders.  The only thing missing from the office is the piece of metal.  Bannerman still has the pages of notes since Sanders was unaware that they existed.  Bannerman doesn’t know why Swendler gave him the notes, but he thinks it may have something to do with Aegiria.  He decides that he and his young aide, Peter Blake (Stewart Guidotti) should go to Aegiria to check it out.  Bannerman, Peter and Dr. Deraad (Richard Coleman) all board the submarine, Cyana.  Also on board is Janet Slayton (Ingrid Sylvester).  Janet won an essay contest.  The prize was a trip to Aegiria.

Sir George is not happy that Bannerman is going to Aegiria and taking the notes with him.  He orders Sanders to find a way to stop the submarine from leaving.  Sanders hires a tugboat to ram the sub hoping that it will be incapacitated enough to require that she return to port for repairs.  The sub leaves earlier than expected so when the tug hits the Cyana is does minimal damage.  Captain Payne (Peter Williams) decides to go on to Aegiria and make repairs there.

Dr. Deraad (Richard Coleman) is a frantic hot mess.  His strange behavior comes to the attention of Janet.  She also noticed that Deraad had a piece of metal that was shaped like a disk and was furious with her when he realized she saw it.  Janet mentions it to Peter.  He tells her to talk to Bannerman in the morning.  Dr. Deraad overhears their conversation.  Hatching a diabolical plan, he arranges for Peter and Janet to meet in the compartment of the submarine that was damaged by the tug.  He sabotages the braces that were shoring up the damage and locks them in.  When the sub goes deeper the hull springs a leak.  Peter and Janet have no way out.

Technically you don’t have to watch “City Beneath the Sea” to enjoy the sequel but it does give context to some of the situations that could be a little confusing without it.  They are both part of a British loose seven series children’s science fiction programming venture.  The series started with “Target Luna” 1960, a lost series, “Pathfinders in Space” 1960, “Pathfinders to Mars” 1960, “Pathfinders to Venus” 1961, “Plateau of Fear” 1961, a lost series, “City Beneath the Sea” 1962 and “Secret Beneath the Sea” 1963.