The story opens with Nancy tending her prize delphiniums when a mysterious carrier pigeon lands in her yard, causing her to contact the registry for the birds. Meanwhile, housekeeper Hannah Gruen takes a fall and must be treated at the local orthopedist's office. The attending physician, Dr. Spires, later confides to Carson Drew and Nancy that he was forced to tend an elderly woman for her shoulder - the drivers of a car blindfolded him when they drove him there, so he wouldn't be able to guess her location, leading him to believe the patient was a prisoner. The only clue to her identity is a bracelet with a family crest, and the doctor's belief that she was being held on Larkspur Lane. Nancy, of course, immediately sets out to track the crest, discovering that it belongs to the Eldridge family of St. Louis.
In the meantime, the registry contacts Nancy about the pigeon, suspecting something criminal is involved. Effie Schneider, a foolish girl, is serving as Hannah's substitute, and gets in on the action when Nancy attempts to track the carrier pigeon's flight into the country. A strong woman attacks Nancy downtown, then steals the bracelet from her. Due to threats of intruders and increasing danger, the Drews accept Helen Corning's invitation to visit Sylvan Lake.
Coincidentally, Nancy rescues a young Eldridge child from a mishap at the lake, and learns that an elderly relative is missing. Nancy and Helen explore the vicinity, finally finding "L.S. Lane." Near the little-used road is an estate surrounded by an electrified fence, and also surrounded by delphiniums. Elderly patients are outside on the grounds. Mrs. Eldridge is outside, near the fence, and reveals that she is indeed a prisoner. When involved in a struggle outside, Nancy is knocked-out after being thrown to the ground. When Nancy wakes up Bess warns her 'Nancy you were knocked-out yesterday, don't get into more trouble'. However, Nancy proceeds with the case. Nancy and Helen disguise themselves as an old lady and her nurse, and enter using the password, "singing horses." Nancy reaches Mrs. Eldridge, rescues her, and sends her safely away with Helen before being imprisoned by the evil doctor and his partners, who are kidnapping wealthy elderly people and forcing them to sign over money and securities. She escapes from her cistern prison and sabotages their getaway airplane, just as the police arrive.
The plot is very similar, although condensed, and Bess and George are included in more of the action. "The" is dropped from the title. A subplot involving supernatural events at Helen Corning Archer's in-laws' summer place on Sylvan Lake (ghostly blue wheels of rolling fire) leads Nancy there. The butler, Morgan, is involved in some minor crimes as well. Bess accompanies Nancy in the rescue operation instead of Helen, with Bess dressing like a nurse, and George is injured.
Russell H. Tandy's original dust jacket artwork remained in print until 1962, long after most early volume dust jackets had been modernized for 1950s readers by illustrator Bill Gillies. The original art shows Nancy in a genuflection position wearing a very full, loose dress. Collectors speculate that publisher Grosset & Dunlap commissioned an updated illustration of the same scene during the transition from Gillies to new series artist Rudy Nappi in 1953. However, due to the presentation of Nancy in a pencil skirt, in a crouching pose,the new painting showed an indiscreet display of part of her thigh (the slim skirt has crept above Nancy's knees). This was deemed inappropriate for American readers, and the artwork was shelved. The art later appeared on British dust jackets for this volume in 1960.
In 1938, the story was used as the basis for Nancy Drew, Detective, the first of four Nancy Drew movies, starring Bonita Granville.
- This is the last book that Walter Kirag wrote before Mildred A. Wirt decided to return to write for the Nancy Drew Mystery Stories series.
- Due to Walter Karig dying in 1956, the original text of this book is in the public domain in Canada, while it is still under copyright in the U.S. until 2028. In Canada, even for "works for hire", the length of copyright depends on the life of the author plus 50 years after the year in which the author passed away.