The band exists in the space between what they promised and what they're terrified they can't deliver. There's no villain here, no external obstacle, just five people staring down the weight of their own word, watching autumn turn and realizing they have to become different versions of themselves to make good on it. Lisa writes lyrics about promise and discovers she's already changed. Yukina and Sayo leave letters to the people they need to become. Rinko composes because the band needs her to, not because she has something to prove. The movie understands that the hardest promise isn't the one you make to an audience, it's the one you make to the people standing next to you, and the only way to keep it is to stop being who you were.

Roselia stands at the edge of autumn, five musicians bound by a single promise: to perform at FUTURE WORLD FES. Each member carries their own weight as they push toward that distant stage. Lisa has grown through writing the lyrics of "Promise," while Yukina and Sayo, watching the autumn leaves change color, set down their resolve in a picture letter to their future selves. Rinko, wanting to do something for the band, begins composing a new song. Meanwhile, Ako grows anxious as she tallies the remaining goals ahead: a contest, a tournament, and the festival itself. The question hanging over all five is what they will see once FUTURE WORLD FES is behind them. The film is a music drama centered on the band at this moment.