Two years have passed since we last visited the Little family and much has changed. Stuart has blossomed and settled nicely into his new life. In addition to his big brother George, Stuart now has a new baby sister, Martha. Even Snowbell, the curmudgeonly cat with an attitude, has warmed up to Stuart in his own way. Yearning for a friend of his own - someone like him, Stuart haphazardly saves an injured bird, Margalo. When Stuart learns that Margalo doesn't have a place to live, he insists that she stay with him and the Littles. When she mysteriously vanished, Stuart and Snowbell set out on a heart-pounding journey to find her in New York City and unlock the mystery.