Mark Bannerman (Gerald Flood) is journalist and the editor of a science magazine. He and his part time assistant, Peter Blake (Stewart Guidotti) are at a convention for the International Society of Communication Engineers. At a social gathering they meet Professor Westfield (Hayden Jones). Westfield is the inventor of a new communication system for submarines that uses sound. While they are talking, they are interrupted by a man who calls himself Swenson (Denis Goacher). With the appearance of Swenson, Professor Westfield’s demeanor changes. He seems uncomfortable and more reserved. Not long after that Mark and Peter leave the convention. Westfield is about to say something to Mark when he is again interrupted by Swenson.
The next day Peter sees in the paper where Westfield has disappeared. The paper notes he was last seen with Swenson. Mark says that Swenson looked familiar. Eventually they figure out that Swenson is not really a communications engineer from Stockholm but is really Captain Kurt Swendler, a former U-boat captain, that was reported missing at the end of WWII. They notify Captain Payne (Peter Williams) at the Admiralty.
Payne invites both Mark and Peter on the submarine Cyana where they will be testing Professor Westfield’s invention. They get a quick tour of the sub and are shown the Professor’s communication system. At that moment another submarine surfaces, this one fully armed. Piloting the other sub is Swendler. Swendler has the crew of the Cyana set adrift and takes over the British sub, unaware that Mark and Peter are aboard and hiding.
After Gerald Flood and Stewart Guidotti went into space in the Pathfinder television series they were tapped again, this time for “City Beneath the Sea”. The series was released in 1962. It consisted of seven episodes. A sequel, “Secret Beneath the Sea”, was released a year later with six episodes. Gerald Flood and Stewart Guidotti reprised their roles for the sequel. Peter Williams, who played Professor Wedgwood in “Pathfinders in Space” and “Pathfinders to Mars”, plays Captain Payne in the “Beneath the Sea” films.