|The following information is from the Adventure Time comic book series. It may not be part of the same canon or continuity as the animated series.|
Issue 31 of the Adventure Time comics.
"When Princess Bubblegum and Marceline get their driver's licenses, they have the perfect plan—build a giant ramp and drive off of it! It's gonna be so much fun...wait, who's the Mnemonoid, and why is he all up in this comic?"
Princess Bubblegum and Marceline have pass their driving exams and have received their licenses (even though they have been driving for a long time without them, they just want the police from other kingdoms to stop hassling them).
Right as Princess Bubblegum thinks of an idea they can do with their licenses, the comic is interrupted by an alien. The alien, calling himself the Mnemonoid, claims to have taken over the reader's mind and is projecting his image over an allegedly blank page. After introducing himself, he lets the reader return to reading the comic but promises that they'll meet again.
On the next page, Princess Bubblegum and Marceline have constructed a racetrack for them to compete on (after having an argument about on it on the previous page). Meanwhile, Finn and Jake are at the Tree Fort, preparing to eat an Everything Burrito, until Princess Bubblegum and Marceline veer offtrack and crash through their wall. Bubblegum and Marceline explain that they just got their license and bought cars to celebrate. This causes Finn and Jake to reminisce about the time they fixed a truck with Banana Man. Princess Bubblegum and Marceline offer to take them for a ride but they decline in order to finish their burrito.
After Bubblegum and Marceline leave, Finn muses on getting a license when he's older while getting ice cream for Jake. Ice King then bursts through their roof, shouting about getting payback on the two. After they subdue the Ice King, he explains that they had offended him by not coming to his birthday party. Finn and Jake never received the invitation because Guntor never delivered it. As an impromptu birthday gift they give him a note saying "happy birthday", much to Ice King glee.
At that point, the Mnemonoid bursts in declaring he's going to steal from Finn and uses to his psychic powers to restrain everyone but Finn. Luckily, Princess Bubblegum and Marceline race in and land their cars on him. Unfortunately, the Mnemonoid uses is powers to push the cars off him and grabs Finn to take what he wants.
Finn then wakes up in his bed to find that five years have past.
Back up story
Lemongrab looks in the mirror one morning and decides he wants a new face. He runs off and grabs Cinnamon Bun and ties him to his head declaring him to be his new face. Lemongrab bursts into the Tree Fort to show off his new face. Finn and Jake rescue Cinnamon Bun and Lemongrab starts screaming until Jake draws him a new face on the back off his head. Finn and Jake go adventuring while Lemongrab stays and poses for hours in the Tree Fort until he falls asleep. Ice King shows up in the middle of the night, hoping to get Finn and Jake to have a cheese fondue with him, when he sees Lemongrab's new face. Believing it to be a new "Lemon Princess", he begins sweet talking it and then kisses it, causing Lemongrab to wake up and freak out. He orders the depressed Ice King to leave and gives up trying to get a new face.
Comic preview pages and panels