The Bobbsey twins ;

The Bobbsey twins help the members of a Camp Fire club catch the culprits who stole their bicycles.

A scarecrow who disappears and reappears at other locations and an odd-shaped key lead the Bobbsey twins into another mystery.

When a valuable stamp is stolen from a museum, the four Bobbsey twins try to help find the thieves.

The Bobbsey twins are drawn into a mystery involving the manufacture of perfume while on a family vacation in Paris.

The Bobbsey twins suspect that odd events at a historic Oklahoma mansion and the theft of a duplicate dollhouse may be connected with rare animals being stolen from the zoo.

Strange sounds in the attic convince the Bobbsey Twins that there is a ghost, and with Nan and Bert away and their parents skeptical, it is up to Freddie and Flossie to solve the mystery.

From the book:"What are you doing, Freddie?" asked Bert Bobbsey, leaning over to oil the front wheel of his bicycle, while he glanced at his little brother, who was tying strings about the neck of a large, handsome dog. "Making a harness," answered Freddie, not taking time to look up. "A harness?" repeated Bert, with a little laugh. "How can you make a harness out of bits of string?" "I'm going to have straps, too," went on Freddie, keeping busily on with his work. "Flossie has gone in after them. It's going to be a fine, strong harness."

While vacationing in Bermuda, the four Bobbsey twins become involved with a pair of thieves trying to steal valuable artifacts from the local museum.

The Bobbsey twins learn to use a computer and apply their detective skills to a mystery involving a clown, a ghost, and a prize won at a school fair.

Twins Freddie and Flossie take the train to Grandma's house with their cat Snoop.