In the Foster's universe, imaginary friends take physical form and become real as soon as children think them up. Once the children outgrow them, the friends move to Foster's Home for Imaginary Friends, where they stay until other children come to adopt them. The home is run by the elderly Madame Foster, its founder; her friend Mr. Herriman, the business manager; and her granddaughter Frankie, who handles day-to-day operations.
Mac is an eight-year-old boy, whom his mother told that he needs to give up his friend Bloo, who hears about Foster's and thinks it will be a perfect place for him to stay. Bloo moves in with the lanky and one-armed Wilt, the scary-looking but cowardly Eduardo, and the bizarre bird/airplane/tree friend Coco. He is saddened to learn that staying at Foster's makes him eligible for adoption by another child, but eventually Madame Foster makes a special deal: as long as Mac visits every day, Bloo can live there and not be adopted.