Henry, Jessie, Violet, and Benny used to live alone in a boxcar. Now they have a home with their grandfather, and an exciting new exhibit to explore—from behind the scenes—at their local zoo. But disaster strikes when a pair of rare ferrets is stolen. The two huge California condors are the thief's next target. The Aldens quickly decide that the animals are being taken by someone that works in the zoo. But how do they stop a thief who has keys to all the cages? The Boxcar Children are determined to solve the mystery before the zoo looses any more rare animals!