Richard Goulbourn, a humble clerk, comes into a fortune that enables him to start courting the beautiful and rich Moira. He and his sister move into the house next to the black house in Harley Street where Moira lives with her uncle, Dr. van Mildert, her uncle's assistant, Miss Lamotte, and some very sinister servants. But there are undercurrents of evil. Soon the young people find themselves embroiled with an evil mastermind. Kidnappings, ransoms, stolen diamonds, a booby-trapped house, hypnotism, escapees from Sing-Sing... all the elements for a ripping good mystery are present!
