In London a young lady meets a homeless and apparently penniless Russian prince. She introduces him to her middle-class Fulham family and he moves in. It turns out he still has a number of diamonds given him by the last czar, and he is persuaded to start selling them. The resulting money, and his princely notoriety, soon cause changes in everyone's lives.
When a photograph is taken at the scene of a murder, the camera is tossed out of a castle window to destroy the evidence and lands in the back of a passing car belonging to chemist John Gray who becomes amateur sleuth after developing the film and goes in search of the woman captured by the photograph. When the camera is stolen from his laboratory, Gray's suspicions are further...