It's Jeff's 40th birthday, and he's not happy about it. After a night of heavy drinking and a batch of mysterious Korean age-reversal pills, Jeff falls into a coma dream in which he and his friends are members of G.I. Joe, the classic 80's kids' cartoon that encapsulated his youth.
Jeff's dream begins with Cobra forces closing in to destroy the Taj Mahal. G.I. Joe forces, accompanied by Wingman (Jeff), Buzz Kill (Britta), Three Kids (Shirley) and Tight Ship (Annie) defend the target, and Jeff kills Destro in the process. Joe and Cobra troops alike are appalled by the murderous act. G.I. Joe and Cobra have been battling one another for over 20 years, and this is the first murder on record.
Later, at a Joe trial, Jeff attempts to defend his actions by labeling Cobra as a terrorist organization and suggesting that, by not killing Cobra members, the Joes are just contributing to terrorism. The Joe high council won't hear it though, and Wingman, Buzz Kill and Tight Ship are locked in a prison cell. Their cellmate, Fourth Wall (Abed) insists that Jeff is suffering from a psychological break from reality and that they're all actually in a children's cartoon.
Cobra forces, led by Vice Cobra Assistant Commander (Dean Pelton), Lipitor (Hickey), Chang (Chang) and XimXam (Duncan), attack the Joe base and blast a hole in the prison wall, inadvertently allowing Wingman, Buzz Kill, Tight Ship, and Fourth Wall to escape.
That night, Wingman dreams about a toy commercial in which children are playing with G.I. Joe action figures. Fourth Wall insists that this commercial dream is another level of reality that separates Jeff from the real world and keeps him locked in his cartoon coma dream. In order to return to reality, he'll have to break through the commercial barrier.
Meanwhile, the Cobra and G.I. Joe organizations team up, under the new name "Jobra," to rid the world of the murderous Wingman and his band of friends. Wingman and his team head to a newly discovered Cobra excavation site, called "Greendale," to discover the truth. Abed modifies Wingman's jetpack to allow for safe travel through the commercial dimension, which should allow Jeff to make it back to reality. Unfortunately, Jobra attacks at the most inopportune time imaginable.
While Buzz Kill, Fourth Wall, Tight Ship, and Three Kids fend off the Jobra forces, Wingman makes his escape with his modified jetpack. With a little luck and some skill, Jeff travels into the Children's Commercial dimension and escapes his child controller to make it back to reality, where he wakes up in a hospital bed surrounded by his friends.
When Jeff confesses that he drank and pilled himself into a temporary coma, the gang teases him about his real age before assuring him that they love him no matter what. Also, Hickey will always be way, way, way, way older than Jeff.