A boy and his beloved imaginary friend are able to stay together at an orphanage of sorts for imaginary friends that children have outgrown to be adopted by new children.

What happens when children outgrow their imaginary friends? They take them to Madame Foster's Home for Imaginary Friends, a vast Victorian mansion with endless rooms and hundreds of thought-up playmates.

At the heart of the story is Blooregard (aka Bloo), an imaginary friend created by an eight-year-old boy named Mac.