What happens
Clark Kent and Jimmy Olsen, with the captive Professor Browning, discover a hidden room of books on Keno Island, each volume cut with a square hollow. They suspect Dr. Deutsch is using radium concealed in these hollows. When Deutsch has the passageway sealed and the ventilation reversed, the three are left in danger of suffocation. Jimmy collapses from the bad air, and Superman, who is not in the group, works to break them free. Kent and his companions escape through an old tunnel, then overhear Deutsch arranging a radio-controlled plane to carry a shipment of books away from the island. The episode ends with Kent and the others pursuing the plane.
Keno Island, a cavernous island base with secret tunnels and storerooms of books, 1941 radio serial
Characters: Clark Kent, Jimmy Olsen, Professor Browning, Dr. Deutsch, Superman, Hanks, Mr. King
Listen for: The square hollows cut into the books and the reversed ventilation machinery that nearly suffocates Kent, Jimmy and the professor.
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.