Plot

When 8 year old Mac's mother tells him that he's too old for an imaginary friend, he brings Blooregard "Bloo" Q. Kazoo to Foster's Home For Imaginary Friends where he is told that Bloo won't be adopted if Mac visits him every day. Soon after, Mac's older brother Terrence and an evil imaginary friend named Duchess make a plan to try to get rid of Bloo. Mac, with the help of friends named Wilt, Eduardo, and Coco, try to stop them.—Anonymous

In the series premiere movie, Mac sends Bloo to live at Foster's Home for Imaginary Friends after being told by his mother that he is too old to have an imaginary friend. There, they meet Mr. Herriman, Frankie, Wilt, Coco, and Eduardo, who all announce that eventually, Bloo will be adopted by another kid. Mac is promised that Bloo will not get adopted if he visits Bloo every day, but they face problems when Mac's brother Terrence and a fiendish imaginary friend named Duchess want to get rid of Bloo. Now it is up to Mac and his new best friends to join forces and rescue Bloo. They fight an imaginary monster called an Extremosaurus (which resembles a giant spider). The Extremosaurus chases Bloo around the junkyard, like in Pac-Man, but with the help of Eduardo's bravery, Coco's eggs, Wilt's basketball skills, and Mac's intelligence, they rescue Bloo.

Eight-year-old Mac and his imaginary friend Blooregard Q. Kazoo (or "Bloo" for short) often get into fights with his 13-year-old brother Terrence. When Mac's mother tires of this behavior, she tells him that he has outgrown his age to have an imaginary friend and must get rid of him. Crushed by overhearing their argument, except for Terrence, who is rather pleased, Bloo later comes across a TV commercial for "Foster's Home for Imaginary Friends"--"where good ideas are not forgotten," according to the motto.

The next day, Mac and Bloo stop in at the sprawling mansion and are met by Mr. Herriman, the strict business manager. After Bloo explains the situation in comically exaggerated detail, they are given a tour of the house. Frankie, the caregiver, is about to show Mac and Bloo around; however, she is soon called away by the ill-tempered, high-maintenance resident Duchess. Basketball-loving Wilt takes over the tour and introduces Mac and Bloo to the wide variety of imaginary friends that live in the house. Along the way, they meet Coco, who lays plastic eggs when she gets excited and only says "Coco" when she speaks, and the fearsome-looking but soft-hearted Eduardo. Mac and Bloo both think Foster's will be a good place for Bloo to live. However, Frankie tells them that if he stays there, he will be eligible for adoption whenever Mac is not around. Mac promises to stop by after school and departs, taking Coco's eggs with him, leaving Bloo alone with his new housemates who show him their bedroom where he will be sleeping at. Seeing Bloo about sleep on the floor, Wilt lets him take his bunk in exchange for sleeping on the floor and they all fall asleep for the night.

The next day, a wealthy rich couple stops by Foster's to find a friend for their spoiled daughter. They only want the best for her, and Frankie sees a perfect chance to get Duchess out of the house for good. The married couple agrees. Just as Mr. Herriman is getting ready to do the paperwork for the adoption, though, their daughter catches sight of Bloo and starts chasing him. Wilt, Coco, and Eduardo race all over the house to keep Bloo out of reach, but the married couple's daughter finally snatches him away and shows him to her mom and father. They agree, but only Mac's last-minute arrival saves him. The millionaires leave empty-handed, while Duchess becomes even angrier at not being able to leave Foster's, which she refers to as a dump, due to Bloo's interference.

Terrence, meanwhile, has been watching from behind the bushes across the street and realizes that Mac has not gotten rid of Bloo. He and Duchess join forces to do away with their common enemy. As Mac is on his way to Foster's the next day, Terrence keeps him from reaching Foster's, carries him back home, and locks him in the bedroom closet. Terrence then pays a visit of his own, dressed to make a good impression. Duchess creates a diversion by provoking one of the Extremeasauruses (dangerous monster friends created by teenagers), leaving Bloo alone with Terrence.

Mac finds Coco's eggs in the closet and gets some tools from them needed to make his escape. He is too late to stop the adoption from going through, but he and the others soon realize that Terrence and Duchess are working together. That evening, Terrence takes Bloo to a junkyard and meets Duchess, who plans to feed Bloo to an Extremeasaurus she freed earlier as revenge for foiling her chance of being adopted. They are foiled by the arrival of Mac and company, who manage to save Bloo and trick the monster into turning on its masters.

Once everyone is back at Foster's, Mac and Bloo are surprised by the arrival of its founder, Madame Foster herself. She announces that Bloo can live there permanently and never be put up for adoption, as long as Mac visits him every day after school, although Mr. Herriman is not happy with this as it defies the house rules. As for Duchess, her punishment is to be forced to stay at Foster's, the place she hates so much, while Terrence finds himself at the mercy of a herd of annoyed pegasi, whom he had taunted earlier during the junkyard fight.