Pamela Sue Martin portrayed Nancy Drew in the 1977-1978 TV series The Hardy Boys/Nancy Drew Mysteries (1977)
The first mystery that Nancy says she solved was finding a missing girl held captive behind a hidden staircase. "The Hidden Staircase" was the second Nancy Drew book. Also, a movie called Nancy Drew and the Hidden Staircase was released earlier the same year that this episode aired.
Freddie Prinze Jr. originally played Carson Drew in this episode. When the pilot was picked up to series, he was replaced by Scott Wolf and all his scenes were reshot.