This is the fourth of four Nancy Drew films by Warner Brothers starring Bonita Granville. It was based on the Drew story "The Hidden Staircase", though I've never read the book and have no idea how close it is to the original tale. But, knowing Hollywood, many liberties were taken with the story.
The story begins by learning that two elderly sisters, the Turnbulls, stand to inherit the home they've been living in many years. However, the will has an odd proviso--that they MUST remain in it every night until some goal is met. Well, they've abided by the will so far, but when their servant is murdered and all sorts of mayhem follows, the sisters' resolve begins to wane. Naturally, the pushy Nancy Drew has decided to make the case her business and naturally the cops investigating are all idiots!
This film is one of about 600,000 B-mysteries made by Hollywood...and they were churned out like mad by both the big studios and the tiny ones. Why? Well, they were pretty cheap to make and made plenty of money! And, compared to the rest of them, this one is a bit better due mostly to better and more competent direction. Too many of them simply looked rushed but this one seemed more polished and complete. Well worth seeing.