Yukina Minato gathers members intertwined with different thoughts to form a band and participate at the Future World Fes. This is the story of their "promise" from their formation to the challenge of the music festival.

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. What view will the five of them see beyond FUTURE WORLD FES?