When a large, dangerous imaginary friend begins to destroy the city, Imaginary Man comes to stop him. When Mac becomes his sidekick, Bloo becomes jealous and joins forces with Imaginary Man's enemy and sister, Nemesister.
Everyone begins preparing for the annual house photo, and Mac and Bloo try to solve the mystery of a strange photo taken years ago. Meanwhile, Coco tries to lose weight, Eduardo gets a makeover from Duchess, and Wilt tries to fit in the photo.
Bloo is left at home while everyone goes to the beach so he won't ruin the fun, and he and Mac try to have as much fun as possible before everyone gets home.
Old Man Rivers lives across the street and hate's Foster's. On Adopt-a-Thought Saturday, his grandson, Young Man Rivers, annoys Bloo by making faces at him. Madame Foster and Bloo throw adoption forms at him and he keeps the papers. Eduardo must keep the parents busy while Bloo tries to get the adoption forms back.
The gang arrives at Mac's apartment for an unplanned slumber party on a school night, and Mac tries to keep them quiet so that they won't wake his mom.
After Frankie leaves the keys in the bus, Bloo takes Mac on a joyride and gets lost. Wilt promises to keep it a secret, but has trouble not telling anyone.
Foster's needs to get ready for a news show to promote themselves. The only problem is that Cheese keeps annoying everyone to the edge of insanity. So, Mr. Herrimen installs a new alarm system and messes up the code. Cheese is the only one who knows this code!
After Madame Foster rescues an imaginary friend, she is treated as a hero by everyone. Bloo also wants to be a hero so he founds his own home for imaginary friends. Mac tries to make Bloo stop since his home is nothing but a cardboard box and the only "imaginary friends" are two puppets that Bloo calls 'Purple' and 'Sharkhead'. Bloo mistakes a cat for an imaginary friend and makes a family adopt it. Soon Bloo remembers the great times he had with the cat and goes to the family's house to get it back.
Mac's teacher catches a student using an imaginary pen to cheat, and Mac and Bloo save him from the closet. At Foster's, the pen and a green ogre named Moose try to take over the house and turn it into a casino.
In her sleep, Goo accidentally creates all the villain characters from a make believe game that she had helped out on while the power was out at the home during a thunderstorm, who then imprison Mac and Bloo at the home. Now, the gang must finish the game to save their friends.
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 and the other friends start a car wash to pay for a porcelain poodle that Bloo broke. To help, Bloo throws mud at every car that goes by to get more customers.
Mac and Bloo have a staring contest and Eduardo tries to talk to them only to think they are frozen solid, consequently getting Frankie to tell him about it.
Frankie tells Bloo that static electricity can be produced by rubbing socks together. After learning this, he zaps Frankie, then Wilt, then both Wilt and Frankie at the same time. Running through the halls, he zaps everyone.
Bloo is determined to win a game of Hide and Seek, so he hides in the trash. Things go awry when Flutter Nutter and Bingo start playing "Kick the Trashbag".
