Nancy becomes involved in a double mystery concerning a haunted bridge and jewel thieves.

Blue bells will be singing horses! This strange message, attached to the leg of a wounded homing pigeon, involves Nancy Drew in a dangerous mission. Somewhere an elderly woman is being held prisoner in a mansion, and Nancy is determined to find and free her. Meanwhile, the young detective’s close friend, Helen, begs her to solve a second mystery. Helen’s grandparents, the Cornings, are frightened by a sinister wheel of blue fire that appears after dark in the woods outside their home at lonely Sylvan Lake. When Nancy discovers the significance of the eerie signal, she also learns that her two mysteries are connected.

Nancy and her friends Bess and George spend an exciting week exploring a mysterious zigzag house with its fantastically crooked staircase, its bizarre serpent picture, and an unpredictable robot that nearly causes the young detective to lose her life. But despite the threat of danger from the robot, Nancy is determined to solve the mystery of the weird house and to locate the missing owner who is wanted by the police. It takes keen logic and quick thinking for the young detective to plow through the tangled thicket of clues and find the key to this complex puzzle. With the help of her friends, Nancy captures an elusive swindler and uncovers the secret of the crooked banister, but not before they have several hair-raising adventures – one on a broken bridge over flaming water, another in a hidden room with poisoned portraits!

The Hidden Window Mystery: Nancy and her friends, Bess and George travel to Charlottesville, Virginia in search for a missing stain-glass window.

Frank and Joe solve the mystery of the secret locked room in the spooky Dalrymple Mansion.

With only the slender clue of an arrow-shaped tie clasp, Frank and Joe Hardy pick up the trail of a cunning gang of thieves responsible for a wave of jewelry-store holdups. But their investigations are interrupted when a desperate plea for help comes from their widowed cousin who lives on a cattle ranch in New Mexico. Frank, Joe, and their pal Chet fly there immediately, and manage to put an end to the trouble at the ranch and solve the jewelry-store robberies.

Nancy and her friend Helen visit their friend Emily Willouby at the Lilac Inn, which Emily now owns, to help her plan her wedding. Emily plans on selling inherited diamonds in order to help fix up the Lilac Inn. However, Nancy soon learns that someone has been impersonating her and making expensive purchases under her name. Soon after, Emily's diamonds are stolen! Can Nancy find the thieves and recover the missing diamonds?

When thieves hijack a collection of stuffed animals from a country auction, Frank and Joe Hardy pursue the getaway car and are drawn into a thrilling mystery. At the same time, the young detectives’ father—famed private investigator Fenton Hardy—is tracking down an industrial spy ring. This suspense-filled story of pursuit and detection will keep the reader breathlessly following the chain of unexpected developments that lead Frank and Joe to the spine-tingling climax in the wilds of Northern Canada.