"The Case of the Frightened Lady" is from a story from the British writer Edgar Wallace, a man who specialized in writing mysteries and horror stories. Many films have been made from his stories...mostly, but not exclusively in the UK.
The story is set at the mansion owned by an old, rich and well respected family, the Lebanons. Their chauffeur is strangled and naturally the authorities are called in to investigate. Soon they realize that this is a strange home....run by an imperious mother and by servants who never seem very honest nor trustworthy. Yet despite the presence of the police, more killings occur and the trail gets quite strange and murky.
Apart from the cliche of having the Scotland Yard detective having a complete idiot for an assistant (an odd but very common trope from this era), this is a smashing good film. It has some clever twists and fine acting. While I did figure out the mystery myself, it still was satisfying and I am very happy I saw it. Well worth your time.