|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.|
Adventure Time #56 is the fifty-sixth issue of the Adventure Time comic book.
Finn and Jake look for BMO in order to break free of the fantasies they're trapped in!
The comic begins with a look at prior events. In the tree fort, Finn beats Jake at a video game where a sheriff and a bandit spit at each other. Jake thinks the game is rigged since the bandit never wins, and BMO confirms this to be true. Finn and Jake decide to go look for bad guys in the real world and also visit a pho place at the Soup Kingdom. BMO thinks they didn't like the game and apologizes, but Finn says not to worry about it and leaves. BMO bemoans that Finn and Jake don't want to play games anymore. Neptr suggests they talk about their feelings, although BMO isn't too thrilled.
BMO then realizes that they need to change their batteries, but finds that there aren't anymore. They prepare to go out and get more. Neptr asks to come along, despite not running on batteries, and BMO lets him join. The two travel to a far-off, abandoned electronics store. Neptr thinks they're scavenging at first, but BMO shows him a huge pile of batteries in a cave and starts gathering some. Neptr questions where the batteries came from, and BMO guesses that the source is some ooze in the corner. The ooze then calls to them (they think it's the bowl it's sitting in) and throws a battery at them. It tells BMO that they had a desire. Neptr thinks it's dangerous, but BMO tells him to hush. The ooze states that it is primordial and potential, but it is only able to create energy to go into the universe, explaining the batteries. It offers to help BMO and asks them what they want, to which BMO says that they want their friends to like their games more. Transforming into a pair of batteries, the ooze offers its power so they can fill the world with imagination and play. BMO removes their batteries and allows the ooze to enter their body, leading to BMOWORLD.
In the present, this was all Neptr's exposition to Finn. Finn wonders what they can do, as talking to BMO causes the world to get changed. Neptr says he has a solution, which Finn assumes is to remove the evil batteries. Neptr says doing that might hurt BMO, and they should actually go to the core and do a system restart. He tells Finn that even when the play worlds are reset, the real world still exists below, and they need to go downward. Oddly conveniently, they find a submarine. They enter the vessel and find Mr. Cupcake and other candy people in the role as crew. Finn plays along and pretends to be captain, commanding them to dive. Soon afterwards, Mr. Cupcake informs him that werewolves have breached the sub. Neptr says that BMO is mixing up genres in an attempt to please Finn and throws silver pies to stop the werewolves. Finn asks how Neptr knew to do that. Neptr tries to answer, but Mr. Cupcake cheers that he saved them again, which puzzles Finn. The sub suddenly stops, as they have reached their destination. Finn and Neptr go to the core, where they find Marceline. She tells them that BMO's parts are mixed up with old magic and that she's been studying them for a while. Finn tells her that Neptr says they just need to reboot the system, but Marceline worriedly says that's a bad idea. Before she can say what will happen, Neptr starts the reboot, apologizing and saying he doesn't want to leave BMOWORLD.
The comic reveals that Neptr lied a bit and tells the true story behind BMOWORLD. Outside the tree fort, Neptr shows a stick to Finn and Jake, saying it's an evil witch's magic wand. He then breaks it and calls himself a hero like Finn. He says he was playing and asks that they like playing games with robots like him and BMO. Finn says that BMO has cool games, but he has to be in the mood to prank people with pies to play with Neptr. As Finn and Jake leave, Neptr sees BMO going to get batteries and is allowed to follow. At the electronics store, the ooze calls to Neptr, and BMO says it talks sometimes. Neptr talks to the ooze, but BMO thinks it is "bad temptation juice." The ooze knows that Neptr wishes to be more liked and offers to make him a hero. As it is unlimited energy without direction, it turns into a pair of batteries. Neptr opens up his microwave door, assuming the ooze would be going inside him. The ooze says it actually wanted to go inside BMO, so Neptr puts it in them, leading to BMOWORLD.
Back to the present, Finn wakes up in the cowboy world, upset that everything got reset. Jake, thinking he's a cowboy again, wakes up and asks what he's talking about. Finn tells him that they're stuck in an unending computer simulation, but Jake doesn't understand. Finn yells at and shakes the dog, making him remember. Jake asks what they can do if BMO keeps resetting the world, and Finn knows what to do. After scaling the wall to reach BMO, Finn states they need to take out the batteries.