The Mysterious Key and What It Opened

A stranger visits Richard Trevlyn and during a secretive meeting between the two men, his wife Lady Trevlyn listens at the door. She regrets eavesdropping and later when her husband is found dead, she is left not knowing what caused his sudden death. All that might explain it is a mysterious key but no-one knows what it opens. Years later a young boy called Paul befriends the Trevlyn family and in time they come closer to solving the mystery.