At the library, Finn learns of a mountain with treasure possessing transformative properties at its peak. He and Jake venture to find the treasure, but find more than they expected when they encounter the hermit guarding the treasure.
T.V., still living with his mother, discovers a magical object and without knowing, attracts Lee - a rainicorn from the Crystal Dimension, which reveals an important part of Lady Rainicorn's past.
Princess Bubblegum feels sad that Cinnamon Bun is the only candy person in the Flame Kingdom, so she creates Bun Bun, a tiny cinnamon bun, to keep him company.
Consequences regarding Susan's last sent message appear at the Candy Kingdom's shore, bringing about new questions and mysteries leading Finn, Jake and Susan to embark on a new adventure.
After a mysterious craft lands in Ooo, Finn, Jake, Susan Strong, set sail for the Island that it originated, hoping to learn the truth about the past and what happened to the humans.