What happens
Jerry and Joan are left alone in a storage closet nine levels below the surface of Euclidea, the artificial island in the South Pacific, after their friend the Euclidean Girl Submarine Commander departs. Captain Bradford remains unconscious in the prison cells, and Mrs. Gregory stays with him. The two young visitors explore the steel corridors, which all look alike, and try doors that open to reveal hidden mechanisms. They find a transparent steel door and realize that some doors are electrified and that their movements may be monitored. Nearing the center of the island, they come upon an elderly man who is deaf yet tests musical notes on glasses by feeling their vibrations. Jerry hears one sustained note that hurts him, and Joan explains that it is a keystone note with destructive potential. The episode is part of the Magic Island series.
Euclidea, an artificial island in the South Pacific, nine levels below the surface, in its steel corridors and chambers near the central elevator shaft
Characters: Jerry, Joan, Captain Bradford, Mrs. Gregory, Euclidean Girl Submarine Commander
Listen for: The keystone note and the old deaf man testing musical notes on glasses, and Joan's explanation of how such sounds could destroy buildings and aircraft
Written from the episode itself; it never gives away the ending. Every recording is remastered for the stream: cleaned of rumble, hiss and mains hum (and the old TV whine where we found it), then levelled in two passes to steady broadcast loudness, so one show is never quieter than the next.