The issue is told from the perspective of a red gem named Carl. 65 million years ago, during the dinosaur age, a meteor shower happens and one of the meteors hits a volcano and sinks into the mantle where the heat and pressure turns it into Carl. His friend, a blue gemstone named Gemma, was supposed to land with him, but she accidental fell into another volcano. They would stay separated for millions of years waiting for tectonic movement to move them to the surface.
In Ooo, long ago, Bonnibel Bubblegum is tunneling below the ruins of an old city (basically recreating mining), where she locates the gem and when it starts raining, she calls Marceline to pick up when it. Bored, she starts playing "Buns Punch 2" on 3MO, until Marceline comes and sees her playing the game (much to Bonnibel's embarrassment). As they fly off, Bonnibel reveals that she found Gemma and is planning to turn the two gems into friendship amulets for the two of them. She also reveals that Carl can shoot lasers, while Gemma can bring robots to life. Marceline drops Princess Bubblegum off at a work sight where she has robots building her "eternal kingdom." Marceline leaves and goes to the Tree Fort, where she puts her amulet in a chest and calls Bonnibel, who reveals that the she's having problems with Ooze Monster attacks but has a plan to deal with them and all she'll need is a little candy.
A couple hundred years later, Marceline is angry because she feels that the now Princess Bubblegum has abandoned their friendship and is destroying her room in a rage until she knocks open the box with Carl in it and becomes saddened. She picks up the old walkie-talkie the two used to use to relay her feeling on the situation, saying that she knows that Bubblegum has more responsibilities now, but hopes they can find a way to keep their friendship going and promises to help with the construction of the Candy Kingdom. Unbeknownst to Marceline, a teary eyed Princess Bubblegum heard everything.
In present day, Finn and Jake discover Carl in a treasure box Marceline left in the attic before she moved. However, Ricardio, with his limbs repaired, bursts in and takes the treasure chest, proclaiming that the chest has the "Power of Paltime" and taking its essence will make Princess Bubblegum want to hang out with him. After subduing the two with Carl's laser, Ricardio uses Gemma to bring the Tree Fort alive, revealing that it is, in fact, a giant robot (Princess Bubblegum built it into the house for Marceline protection without her knowledge). Ricardio commands Robohouse to beat up Finn and Jake and take him to the Candy Kingdom. Robohouse violently throws them out the window, but are caught by Marceline and Princess Bubblegum who tracked Ricardio here after he stole Princess Bubblegum's amulet. Ricardio order Robohouse to attack the group, but it has a fail-safe that prevents it from harming Marceline. Marceline bursts into Robohouse and reprograms the fail-safe to apply to all of them while Princess Bubblegum and Finn beat up Ricardio, take the amulets back and cut off his arms. Marceline then has Robohouse hit Ricardio so hard he flies right back into Ice King's chest.
Marceline suggest that they make the amulets symbols of the friendship of the four and Finn and Jake suggest they put them back in the treasure chest and put it in a "heroes only" dungeon that they'll build. So the four built the dungeon and Carl and Gemma are finally happily reunited. The Finn, Jake, Marceline and Princess Bubblegum still came back to add treasure to the treasure chest at time, growing older with every visit from Carl's perspective.
At the story's end, it is revealed that it is the far future and Carl and Gemma was telling their life stories to a young female adventurer who managed to make through the dungeon and claim the treasure.
