When a large and dangerous imaginary friend is destroying the city, a superhero friend named Imaginary Man comes and saves the day, leaving Mac impressed. But once Mac becomes Imaginary Man's sidekick, Bloo becomes jealous and joins forces with Nemesis, Imaginary Man's mortal enemy and sister.

The residents prepare for the annual house photo. Meanwhile, Bloo, Mac and Goo search for an answer behind a weird photo of Foster's taken years ago, Eduardo gets an "extreme makeover" from Duchess, Coco realizes she needs to lose weight and Wilt has trouble being in the house photo without getting his head obscured.

When the home's residents go to the beach and leave Bloo in the house alone on purpose so that he doesn't ruin the fun, he and Mac try to squeeze in as much fun as possible and make it the best day of their lives.

Old Man Rivers, who lives across the street from Foster's, does not want imaginary friends on his lawn, and on "Adopt-A-Thought Saturday," his grandson, Young Man Rivers, makes faces at Bloo. Bloo and Madame Foster throw papers — that are adoption forms — at Old Man Rivers, who keeps them for himself, causing Bloo and Wilt to try to get them back. Meanwhile Eduardo tries to keep the parents busy so Bloo can get the adoption forms, but the parents get more impatient by the moment.

Bloo, Wilt, Eduardo, and eventually more guests arrive at Mac's apartment for a slumber party that Mac didn't plan. But because they are all so loud, Mac's mom or Terrence could wake up and get them all into trouble.

Bloo, along with the rest of the house, plan a surprise party for Mac. However, Mac wants no part of it and tries to destroy their plans.

Bloo notices after a trip to the grocery store that Frankie has left the keys in the bus, and decides to take Mac for a joyride in the Foster's bus, and Wilt, Coco and Goo promise to keep it a secret much to poor Wilt's disconcern.

Foster's is going to be on TV, and Frankie wants everything perfect. Meanwhile, Cheese keeps showing up at Foster's, despite being told numerous times that he doesn't live there. In an attempt to keep him out, Mr. Herriman installs a security system. Unfortunately, due to his misreading of the directions, nobody knows the password except for Cheese, who memorized the code as a song. It is now up to the gang to get Cheese to recite it, and craziness ensues.

Bloo alerts that an imaginary friend is in trouble. No one wants to help him at first because it might be a trick, but Madame Foster forces Frankie, Coco, Wilt, and Eduardo to help him. They find that the imaginary friend is stuck in a tree, and Bloo tries to help by climbing on top of them but only causes them to fall. Madame Foster manages to save the friend and is awarded for her good deed. Bloo tries to prove that he can also do good deeds and makes his own home for imaginary friends.

An imaginary pen, that resembles Abraham Lincoln, is caught with a student cheating on a test by Mac's teacher. He is saved from the classroom's closet by Mac and Bloo along with his au pair, a green ogre named Moose. The Lincoln pen cons Madame Foster into a new plan to adopt an overcrowded home to unsuspecting adopters for a fee, then takes over the house, planning to turn it into a casino.

It's time for the 5 Year Creator Reunion Picnic, where imaginary friends' creators come to Foster's to see their imaginary friends. However, once again, Wilt's creator doesn't show up. Wilt, believing that his creator is still mad at him for a past let-down, sets off on a cross-country journey to find an imaginary friend that he lost to in a past basketball game and challenge him to another game to set things straight.

Bloo and the gang are out to find Wilt and discover some interesting things about his secret past along the way.

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.