Wilt is getting tense at Creator Day when Mac keeps mentioning the word "creator" to him. Wilt suddenly leaves in the middle of the night and the Foster's friends look for him. Mac realizes that he accidentally created a new imaginary friend named Cheese. Cheese is allowed to stay at Foster's like Bloo, but Bloo has trouble dealing with his new brother Cheese. Mac and Bloo both develop a crush on Frankie, and try to stop her from having any contact with any guy. Eventually, they follow her on a date while in disguise.

Mac voiced by Sean Marquette is a bright and creative eight-year-old boy and Bloo's creator who visits Foster's every day. However, his extremely good nature tends to make him somewhat naive. He is very attached to Bloo and it is shown that his biggest fear is never seeing him again, because Bloo is what keeps him happy and cheerful and vice versa. Mac becomes hyperactive to the point of a rabid mania when he eats sugar. Once in this state, he becomes impossible to control, will often become obsessed with seeking any other source of sugar.

It was produced by Cartoon Network Studios and is the first show made by the network to be animated primarily with Adobe Flash and was done in Ireland by Boulder Media. Set in a world in which imaginary friends coexist with humans, it centers on a boy named Mac who is pressured by his mother to abandon his imaginary friend Bloo. After the duo discover an orphanage dedicated to housing abandoned imaginary friends, Bloo moves into the home and is kept from adoption as long as Mac visits him daily. The episodes revolve around Mac and Bloo as they interact with other imaginary friends and house staff and live out their day-to-day adventures, often getting caught up in various predicaments. McCracken conceived the series after adopting two dogs from an animal shelter and applying the concept to imaginary friends.