Because his mother believes he is too old for imaginary friends, eight-year-old Mac is pressured by her to abandon his imaginary friend, Bloo. When Mac takes Bloo to Foster's after seeing a television advertisement, they discover that if Bloo were to live there, he would be available to be adopted by another child. Mac then bargains with Frankie, Herriman, and Madame Foster until they agree to guard Bloo from adoption so long as Mac continues to visit the center daily. Mac continues to visit the home every day after school to experience the escapades of the mischievous Bloo and the array of eccentric, colorful characters inhabiting Foster's and the obstacles with which they are challenged.
