When Mac, a shy 8-year-old boy, is forced to give up his imaginary friend, Bloo, he finds the perfect place for him: Foster's Home for Imaginary Friends. There, they meet many of the home's residents, like Eduardo, Wilt and Coco; as well as the home's staff. Their problems appear to be solved until it is revealed that Foster's is an foster home and adoption center, and, if Mac isn't there at the time someone tries, another person could adopt Bloo. To make matters worse, a sinister plot by the evil imaginary friend Duchess, uses Mac's 13-year-old bully brother, Terrence, and puts Bloo in peril. Now Mac and his new friends from Foster's must work together to save Mac's best buddy Bloo.