A fiery glowing eye in a museum, the abduction of Ned, the code name Cyclops, and a pilotless helicopter draw Nancy and her friends into a dangerous investigation.

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.

Kitty and Johnny Blair, two famous actors, adopt baby twins who were mysteriously found in a boat along the river. The cruel Blairs have only adopted the babies as a publicity stunt, as they hope to raise the children as actors, which will help their own careers. Nancy sets off to find their real mother and take them away from the Blairs.

Asked to locate a stolen necklace of unusual value, Nancy soon discovers that strange and dangerous people are responsible for the theft.

Gold! There are rumors that long ago a treasure was hidden in a city now buried under the Nevada desert. Nancy and her friends plan to join a dig sponsored by two colleges to hunt for the gold. Before she starts, the young sleuth receives an ancient stone table with petroglyphs on it. With this amazing clue, however, come a threat and danger from a thief who also wants the treasure. One harrowing adventure after another besets Nancy, Bess, George, Ned, Burt, and Dave in 102-degree temperatures as they pursue Nancy’s hunches above and below ground. They are assisted by a fine woman and a young geology student, but both are unwilling participants in a strange plot. In the end Nancy and Ned nearly lose their lives, just after she has discovered the priceless hidden treasure of gold.

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!

On a student tour through Europe, Nancy discovers that their leader is on a secret mission to transfer ten refugee children from an iron curtain country to freedom! Before the mission is completed, Nancy receives an urgent message from her father concerning a missing entry in a foreign film festival. Undaunted and clever, Nancy pursues an intriguing clue found in a student's wheelchair and finds herself in great danger.