When Mac, a shy 8-year-old boy, is forced to give up his imaginary friend, Bloo, he finds the perfect place for him: Foster's Home for Imaginary Friends. There, they meet many of the home's residents, like Eduardo, Wilt and Coco; as well as the home's staff. Their problems appear to be solved until it is revealed that Foster's is an foster home and adoption center, and, if Mac isn't there at the time someone tries, another person could adopt Bloo. To make matters worse, a sinister plot by the evil imaginary friend Duchess, uses Mac's 13-year-old bully brother, Terrence, and puts Bloo in peril. Now Mac and his new friends from Foster's must work together to save Mac's best buddy Bloo.

When Mac, a shy 8-year-old boy, is forced to give up his Imaginary Friend, Bloo, he finds the perfect place for him: Foster's Home for Imaginary Friends. There, they meet many of the home's residents, like Eduardo, Wilt and Coco; as well as the home's staff. Their problems appear to be solved until it is revealed that Foster's is an adoption center, and, if Mac isn't there at the time someone tries, they could adopt Bloo. To make matters worse, a sinister plot by an evil Imaginary Friend uses Mac's 13-year-old bully brother, Terrence, and puts Bloo in peril. Now Mac and his new friends from Foster's must work together to save Mac's best buddy Bloo.

It's Madame Foster's birthday and Frankie has forgotten the one thing necessary for any party, the streamers! As Mr. Herriman orders her to go get some, Bloo, Eduardo, Mac, Wilt, and Coco tag along as well. They have five minutes left until they can leave, so they stop a one store and Frankie falls asleep in a vibrating chair and when she wakes up, everyone left to different places. Will Frankie be able to get to the party in time?

Bloo finds out about a door in the home that everyone is forbidden to open. As he asks around about the door, no one tells him what is inside of it. However, in the middle of the night, Bloo manages to sneak to the door and open it, but will he like what he has found or will his deeds cause untold trouble?

Mr. Herriman isn't happy about Bloo being able to stay at the home indefinitely, and is getting on the little guy's case about the rules of the house. Unfortunately, when Bloo learns that Mr. Herriman will have him ejected from the home if he doesn't toe the line, Bloo gets so stressed out that he accidentally breaks a statue of Madame Foster! Bloo and his buddies will have to fix the "busted bust" before Mr. Herriman finds out and has Bloo thrown out on the street.

When Mac visits Foster's, Bloo wants to show him something up on the roof, though when it's time for them to go back downstairs, they find themselves lost and unable to get downstairs. With Bloo becoming unstable due to hunger and Mac potentially getting in more and more trouble with each passing minute, will they manage to find their way back down before it's too late?

Mac and Bloo are using Frankie's digital camera to make video interviews for the new Foster's website, when they stumble upon Mr. Herriman indulging Madame Foster's whims and doing a little dance for her. Much to Mac's chagrin, not only do Bloo and Frankie show the embarrassing footage to almost everyone in the house, but Bloo uploads the film to the Internet. It won't be long before Mr. Herriman discovers he's become the latest Internet fad, and you can bet he won't be happy!

As a new day begins at Foster's, with everyone enjoying their favorite television program, The Loved and the Loveless, a new friend named Berry enters the house as sweet as can be, though with one glance on Bloo became love struck and determined to make Bloo love her back, though there is one thing that truthfully stands between her and Bloo, and that is Mac. When Mac and Bloo are trying to win a world record, Berry gets mad. After many schemes to try to get rid of Mac, Berry tries to tell Bloo that Mac is stopping him from winning the world record. When Bloo says that to Mac, he gets mad and dosen't want to break a world record anymore. But Berry makes Bloo the almost world's largest rubber band ball and both of them will put the last rubber band to make it the biggest. Meanwhile, Mac is running to Foster's so Bloo and Mac can put on the last rubber band, but will Mac get there in time before Berry and Bloo put on the rubber band?

Terrence has an idea to make up a friend named Red to beat Bloo and Mac up with, though the friend he creates doesn't seem to be capable of comprehending the task it was given. Will Red serve Terrence as he was created for, or will it choose it's own path and disregard what Terrence ordered him to do?

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 is a friend, he takes him to Foster's, thinking everyone will give him more praise than they give Wilt.

After Eduardo finds a puppy when he was taking out the trash, he decides to keep it, despite the fact that dogs are not permitted inside the house. However, despite Eduardo's best efforts, the dog gets loose and starts causing problems that Eduardo takes the blame for in fear of the puppy being found out. Will Eduardo manage to keep the puppy without Mr. Herriman finding out, or will the puppy be forced to leave due to Mr. Herriman's strict policy against dogs?

It's Adopt-A-Thought Saturday, an event that focuses on getting good friends adopted on the weekends when children are out of school. But Mac and Bloo find out after their good friend Jokey gets adopted. While the event is usually a good thing for both imaginary friends and those seeking to adopt them, Bloo doesn't want his friends to be adopted, so he works with Mac to try to find a way to put an end to the event. From atracting friends in the house with scented cupcakes to dropping plastic spiders, they try as many plans as they can. Will they be successful in prevent the adoption of their friends, or are their best efforts in vain?

Bloo and Mac enjoy playing with each other in the mud while raining, but sometimes there can be consequences. As Mac and Bloo get sick, causing Bloo to turn white and Frankie to take Mac home, Bloo misses out on the scary movie the friends planned to watch that night. However, the movie scares the friends enough that they believe that the ghost is real, and they mistake Bloo to be the ghost. Will they be able to figure out it's him before they end up hurting Bloo or themselves in the process?