It’s a group performance by 4 band members!
The story focuses on human relationship and the regret the members had in the past through their band activities!

Ritsuka is in the 2nd grade of high school (=11th grade). (The one with brown hair.)
One day, he encounters a boy named Mafuyu, who fixes his broken guitar, which Ritsuka has been cherishing so much. Ever since they encounter, they start to get along well with each other, Ritsuka ends up becoming into a band activity. Mafuyu introduces his two friends at his college; Haruki and Akihiko, to Ritsuka, and four of them form a band group.

This is how the story starts. They all have been struggled with the regrets that they had before. They now face their own honest feelings while growing themselves as a group band.