Mark Bannerman (Gerald Flood) and Peter Blake (Stewart Guidotti) have hidden aboard the British submarine Cyana.  As Kurt Swendler (Denis Goacher) and his crew pilot the stolen sub, Peter sneaks into the radio area and turns on Professor Westfield’s (Hayden Jones) experimental communication system.  The radio/receiver sends out a signal that the Royal Navy picks up.  The Navy sends out ships to intercept the sub and take it back.  Before they reach the sub, Swendler discovers that the system is on.  Swendler is now aware that someone else is on board the submarine.

A search of the ship turns up Mark.  Peter continues to hide hoping to cause more problems for Swendler.  As the Royal Navy passes overhead Peter tries to signal their position by tapping on the hull with a spanner.  Peter is quickly found as well.  Swendler finds that there is an ocean liner in the area.  The sub follows underneath so that their position is camouflaged by the liner.  The sub slips away.

Captain Payne (Peter Williams), of the British Royal Navy, does some research and finds that Swendler had been working on a top-secret project when he disappeared.  Several top scientists disappeared at the same time.  The person in charge of the project was Professor Ludwig Ziebrecken (Aubrey Morris).  Ziebrecken also disappeared.

Aboard the Cyana, Swendler takes the submarine to a place he calls Aegiria.  After being in the submarine for weeks Mark and Peter are confused as to where they are.  At first they believe they are on land but eventually they come to realize that they are still underwater and that Aegiria too is under water.     

Both “City Beneath the Sea” and “Secret Beneath the Sea” were written by John Lucarotti.  Lucarotti also wrote some of the “Avengers” episodes and fifteen episodes of Dr. Who.

“Plateau of Fear” was a sort of prequel to the “Beneath the Sea” series.  It was a six-episode British science fiction children’s drama that starred Gerald Flood as the scientific journalist, Mark Bannerman and Stewart Guidotti as his assistant, Peter Blake.  The entire series is believed to be lost.