The Foster's Gang is preparing to spend Christmas in London, gathering at Foster's House in Toonvile suburb on the night before their departure, while Terrance disguised as a cop is in the house, getting information about every house in the neighborhood.
Bloo accidentally ruins Foster's dinner and their flight tickets to London after a scuffle with Wilt, resulting in him getting sent to the attic of the house as punishment. He berates Frankie and wishes that his friends would disappear. During the night, heavy winds cause damage to the power lines; the resulting power outage resets the alarm clocks and causes the Foster's Gang to oversleep. In the confusion and rush to get to the airport, Bloo is accidentally left behind.
When Bloo wakes up one morning, he finds out that the entire house is completely empty of every imaginary friend, including an immobile friend known as "Never Leave Steve". At first, Bloo is initially bored at having nothing to do until Mac arrives. Mac and Bloo eventually find out where everyone has gone and after some disappointment from Bloo, Mac tells Bloo that the both of them have a once-in-a-lifetime opportunity to do whatever they want at Foster's, and, thinking that his wish has come true, is overjoyed with his newfound freedom and begins doing whatever they want, such as jumping on beds and getting into Wilt's belongings, including his BB gun. He then eats junk food as he watches Angels with Filthy Souls, a film that he was prohibited from watching the night before by Mr. Herriman; however, the film's shooting scene ultimately startles them, and they paused the movie.
During the flight, a sleeping Frankie wakes up, worried that the Foster's Gang has forgotten something. She asks Mr. Herriman if they turned off the coffee machine, locked the doors, and closed the garage. Mr. Herriman says that he did forget to close the garage. Frankie, however, thinks it's something else they forgot, then after Mr. Herriman asks what they possibly could have forgotten, Frankie finally realizes that they have left Bloo behind. Meanwhile, Bloo and Mac sled down the stairs in there home and out the front door. After they realization, Frankie is comforted by Foster's Gang, who offers nothing that soothes her guilty conscience. Frankie realizes what kind of bad co-boss she is.