A soul stone is a rare stone that contains a soul. Princess Bubblegum gives Finn and Jake a map and asks them to find the soul stone for her experiment. Braco asks Finn and Jake to let him get the soul stone for Princess Bubblegum because he loves her and wants her to love him.
Braco is a suitor Peppermint Butler chose for Princess Bubblegum. Braco faces many enemies and is injured severely in the process of retrieving the stone. He had to jump into a pit of fire to get a key, which he uses to unlock the container of the stone.
Monsters chase Braco for taking the stone until a larger monster appears asking why he has stolen his treasure. Braco responds by saying that he needs it for someone he loves. The monster then tells him to take the stone.
Once he returns to the Candy Kingdom, Braco goes to give Princess Bubblegum the soul stone, but she no longer needs it. She had created an artificial soul that is more easily manipulated. Braco is extremely disappointed and goes home. In anger, he smashes the stone, freeing the soul inside.