Wealthy Mary Eldredge (Helena Phillips Evans) decides to donate $250,000 (in 2020 purchase-power that would be $4.5 million) to her alma mater, where teenager Nancy Drew (Bonita Granville) is a student. When Mary doesn't deliver the check to the lawyer Carson Drew (John Litel), Nancy's father, and disappears, it seems as if she has backed out of her original intention. And Nancy's classmates turn on her, annoyed that she had gotten their hopes up about getting a swimming pool for the school.
Nancy sets out to find out what really happened. Along the way, with the help of her neighbor and best friend Ted Nickerson (Frankie Thomas), she deals with the kidnapping of her family's doctor, a slow policeman, and an injured carrier pigeon.
At one point, Nancy and Ted hire a plane to look from the air for a house that fits some clues that they have collected. Planning to rescue Mary, they gain entry to the suspected location, Ted disguised as a nurse and Nancy as a widow. They find Mary but, during their attempted escape, all three are captured. Locked in a basement, Ted finds an old x-ray machine and uses it to produce radio and telephone interference, encoding it to send a call for help. Nancy's father and the police get the message and the gang that had kidnapped Mary is caught.
