At a traveling carnival, Kurt Swendler (Denis Goacher) is performing as an illusionist.  As part of his act, he demonstrates a unique metal he calls Phoenicium.  While placing an intense flame on one side of the thin sheet he puts his hand on the other side to show it is cool to the touch.  In the audience are Dr. Deraad (Richard Coleman) and Sanders (Murray Hayne).  Sanders is a small-time hood that works for the chairman of International Metals Limited, Sir George (Reginald Smith). 

Sir George believes that the Phoenicium can be found near the underwater city of Aegiria.  If the piece of metal that Swendler is proven to be actual Phoenicium Sir George plans to drive the United Nations out of Aegiria, locate the Phoenicium ore and mine it.  Such a metal would prove to be extremely valuable in the space program and taking over Aegiria means International Metals would make billions. 

Sanders and some minions try to get the Phoenicium from Swendler.  Swendler manages to get the Phoenicium and some notes on it to science journalist Mark Bannerman (Gerald Flood).  Swendler and Bannerman became associated when Swendler, a former U-boat Captain, worked for the despot that built Aegiria before the UN took it over.  Bannerman doesn’t know why Swendler wanted him to have the notes and metal disk.  Swendler gets hit by a car and is delirious in hospital, so he is no good to anyone.  Dr. Deraad is also on Sir George’s payroll and is planning on going to Aegiria to find out where the ore is located and cause problems for Aegiria. 

Before Bannerman can do anything with the sample of metal Sanders breaks into his office to steal it.  Before Sanders can leave, Bannerman shows up.

“Secret Beneath the Sea” was released in 1963.  It is a low budget British children’s drama and a direct sequel to “City Beneath the Sea” 1962.  Gerald Flood returns as science journalist Mark Bannerman as does Stewart Guidotti as his young assistant Peter Blake.  Also returning is Peter Williams as Captain Payne.  Unfortunately, Denis Goacher, as former U-boat captain Kurt Swendler, is only in the first episode.