Mrs. Wilberforce (Katie Johnson) lives alone in a rickety Victorian house near London’s King’s Cross railway station. She rents a room to Professor Marcus (Alec Guinness), who claims to be a musician, and asks to use the room for practice sessions with his string quintet.
But wait. Professor Marcus and his four associates are in fact plotting an armed robbery and plan to use Mrs. Wilberforce in their dastardly scheme.
