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. Once they meet various residents and get to know Wilt, Eduardo, Coco as well as the home's staff, the problem arises as they realise Bloo may get adopted if he moves in.

An evil imaginary friend sets a sinister plot in motion against Bloo, and Mac and the home's residents must save him.

Mac races to save Bloo from the sinister scheme of the evil imaginary friend.

Mr. Herriman grows frustrated with Bloo's inability to follow the house rules and threatens to evict him. Under pressure, Bloo accidentally breaks a bust of Madame Foster and attempts to repair it before Mr. Herriman discovers the damage.

When Mac visits Foster's, Bloo insists on showing him something on the roof, but the two become lost and cannot find their way back downstairs. Meanwhile, in the dining room, the others secretly sneak food while Mr. Herriman insists they wait for Bloo before starting dinner.

It's Adopt-A-Thought Saturday, an event that focuses on getting good friends adopted on the weekends when children are out of school. Mac and Bloo find out what Adopt-A-Thought Saturday is about after their good friend Jokey gets adopted and spend the day forcing the friends back into the house so that they won't be adopted.

On Madame Foster's birthday, Frankie realizes she has forgotten to buy streamers for the party, prompting Mr. Herriman to send her to the store. Mac, Bloo, Eduardo, Wilt, and Coco accompany her but wander off in the mall and become involved in various mishaps, slowing Frankie down.

Bloo and Mac play with each other in the mud while raining, which causes them to get sick. As Mac is taken home by Frankie, Bloo misses out on the scary movie the friends had planned to watch that night. When Wilt, Eduardo, and Coco begin to believe that the ghost in the movie is real, they mistake a pale Bloo to be the ghost. Meanwhile, Frankie gets locked out of the house in the rain after dropping off Mac and, as she tries to get back in, feels she's being stalked.

Bloo decides to open the one door at the retirement home for imaginary friends that is never, ever to be opened, and the house becomes infested with something bad.

Terrence has an idea to make up a friend named Red to beat Bloo up with so he can bully Mac to his heart's content, though the friend he creates ends up friendly instead of mean and violent.

Bloo is jealous of Wilt getting special attention for the number of abandoned imaginary friends he brings to Foster's, so he tries to rescue imaginary friends just like Wilt does. When Bloo finds a man in a cell-phone suit and thinks he's a friend, he takes him to Foster's, thinking everyone will give him more praise than they gave Wilt.

Mac and Bloo get embarrassing footage of Mr. Herriman while conducting interviews for a Web site.

A new Imaginary Friend named Berry has her eyes on Bloo, and Mac stands between them.

While taking out the trash, Eduardo finds a puppy and decides to keep it, despite dogs being forbidden in the house due to Mr. Herriman's fear of them.