In Aegiria, Mark (Gerald Flood) and Peter (Stewart Guidotti) meet Professor Ludwig Ziebrecken (Aubrey Morris). Ziebrecken is the creator of the underwater city. It was built on a sunken volcano or guyot. He has kidnapped scientists from all over the world and has them help him maintain his creation. Many of the scientists are willingly helping. Dr. Ann Boyd (Caroline Blakiston), a biochemist, is one of them. She came to Aegiria on the Cyana with Mark and Peter.
Since everyone in the city is assigned work Ziebrecken has Peter work in the gardens with Ann. When Ziebrecken finds out that Mark is a science journalist he assigns him the task of writing a series of articles on Aegiria. Ziebrecken has Swendler (Denis Goacher) show Mark around the complex. Hearing that the backup air system is malfunctioning, Mark takes the opportunity to sabotage the main equipment that extracts oxygen from seawater. Mark’s hope is that everyone will have to leave Aegiria until the system is fixed. Once outside the city he may find a way to escape.
Mark’s idea backfires. With both systems down Swendler must go out into the ocean to repair the backup system. He believes the problem was due to a rock fall. Knowing that Mark is responsible for the oxygen extractor breakdown he makes Mark go out with him to move the rocks. While they are outside Ann realizes that one of them has taken an air tank with a faulty valve. Ann and Peter must go out to help but disaster strikes.
A guyot or tablemount is an underwater volcanic mountain with a flat top or plateau. At one time they were above the ocean surface but gradually they sank underwater. Ocean waves and coral build up assisted in eroding the tops. They were named after the 19th-century geographer Arnold Henry Guyot.