There was a boy named Mac who lived in an apartment with his mother and his jerky older brother, Terrence. But there was one other family member who Mac loves the most. His imaginary friend, Blooregard Q. Kazoo. He and Bloo have gone on all sorts of adventures together. But then, the day came when Mac's mother told him that he had to grow up and give up his imaginary friend. With nowhere to go, Bloo now currently resides at Foster's Home For Imaginary Friends. A massive house where imaginary friends live when their owners leave them and wait to be adopted by other kids. Fortunately for Bloo, he doesn't get to be adopted so long as Mac visits his every day. Now the two along with their new friends Wilt, Eduardo, and Coco have even more fun and adventures throughout the house. But soon, there might not even be a house anymore. Because something evil is in the works here inside Foster's. Something involving a really ugly imaginary 'fiend'.