The elderly Turnbull sisters want to donate their mansion for a children's hospital. However, their father's will states that at least one of them has to stay in the house every night for twenty years before they can inherit the estate; there are two weeks left to go. Then some strange things start occurring. A stranger forces his way past Nancy Drew and brazenly searches the Drew house for related affidavits her lawyer father Carson has obtained. Then, the Turnbulls' chauffeur Phillips dies, though it is uncertain if it was a murder or a suicide. The frightened old ladies consider leaving their home. When Nancy recognizes the dead man as the trespasser, she begins investigating, dragging her boyfriend Ted Nickerson into one predicament after another, eventually getting him fired and jailed. When police Captain Tweedy arrests the two sisters for Phillips' murder, their ownership is endangered. Just in time, Nancy and Ted discover a secret passageway in the basement linking it to the neighboring house, owned by Daniel Talbert. Talbert would make a lot of money if a racetrack were to be built on the two properties, but the Turnbulls had turned down an offer to buy their place.
Frederic Tozere as District Attorney's Investigator
Don Rowan as Phillips the Chauffeur
Dick Elliott as McKeever
Jack Mower as Ice Company Dispatcher
Cliff Saum as Burt the Iceman
Production
It was the only film to borrow its title from a book in the series, although the plot was altered substantially. One critic wrote that "the only similarity between the book and the film was the word staircase." Nancy's boyfriend Ned Nickerson became Ted Nickerson, as Ned was considered too old-fashioned, and housekeeper Hannah Gruen was replaced by Effie Schneider, a minor character who had appeared in only a few books as the Drews' part-time maid; in the films, Effie's traits are combined with Hannah's. Nancy's friends George and Bess were eliminated completely, "mystery elements were downplayed, plots simplified, and the romance spiced up." To promote the film, Warner Bros. created a Nancy Drew fan club that included a set of rules, such as: "Must have steady boy friend, in the sense of a 'pal'" and must "Take part in choosing own clothes." These rules were based on some research Warner Bros. had done on the habits and attitudes of "typical" teenage girls.