Before Mr. and Mrs. Little go on the orphanage they are told by their son George to get a little brother as opposed to a big one. They decide to adopt an intelligent young mouse named Stuart, much to the dismay and irritation of their son George and especially the housecat Snowbell. While Stuart and the rest of the family learn to accept one another, Snowbell decides to enlists the help of the local cat gang to rid the house of the new member of the family.