The Little family adopts an adorably spunky boy named Stuart who looks a lot like a mouse. Mr. and Mrs. Little fall in love with Stuart right away, but their older son George isn't so sure. As Stuart tries to win over his new brother with hilarious feats of courage and daring, the family's cat, Snowbell, gets involved with a gang of shady felines who devise a dastardly plan to get Stuart out of the house...permanently!