After overcoming numerous tribulations, the members of Roselia promised to appear on FUTURE WORLD FES. It is now Autumn... Through writing the lyrics of "Promise," Lisa has matured. Watching the change of color in the autumn leaves, Yukina and Sayo expressed their resolve in a picture letter to their future selves. Driven by the wish to do something for Roselia, Rinko has started to compose a new song. "The remaining quests are the contest, the tournament, and FUTURE WORLD FES..." Reflecting on their goals, Ako starts to feel anxious.

To reach the stage of "FES.", Yukina Minato decides to form a band. Each holding their own convictions, the members of the band come together. The five girls now begin their journey to the top as Roselia... This is the story of their "promise" made to each other, from the start of the band to their challenge towards FUTURE WORLD FES!

The world of Lingalind is surrounded by a mystical wall with seemingly nothing beyond its confines. Worshipped as the mother of the land, the wall delivers celestial gift capsules called "Rakuho'' to locations across the continent. Arriving once a month, the capsules contain metallic armbands that allow the wearer to transform into a mechanical being known as a "Briheight." As a result, Lingalind is thrown into constant turmoil by its warring nations, all hoping to strengthen their military prowess by procuring the offerings for themselves. One day, a Rakuho crash-lands in the countryside with an unexpected inhabitant—a mysterious black-haired man. All eyes are set on this strange newcomer, who calls himself "Back Arrow," when he claims to have hailed from a place beyond the wall—a revelation that can potentially unravel Lingalind's entire dogmatic foundation. [Written by MAL Rewrite]