BMO travels to a far corner of the galaxy to save its people from a deadly threat.

Marceline and Princess Bubblegum journey to the imposing, beautiful Glass Kingdom, and deep into their tumultuous past, to prevent an earthshaking catastrophe.

Follows Peppermint Butler, starting over at the beginning, as just another inexperienced Wizard School student. When mysterious events at the campus cast suspicion on Pep, and his checkered past, can he master the mystic arts in time?