The spring of your freshman year in high school.

Joining the concert band club, Guest meets a young music teacher who has just arrived as the advisor.

Though he is stricter about music than anyone and fair to all students, he gradually opens his heart only to Guest, who faces music with such sincerity.

"When you graduate, let's meet as man and woman."

That promise was a first love that no one could ever know about.

However, rumors spread among students and parents who grew suspicious of their closeness, and the relationship between teacher and student developed into a major scandal.

Seto resigned from his teaching position and unilaterally broke up with Guest to protect them.

"Never look for me again."

Believing you were betrayed, you resolved to forget him through your tears.

Ten years have passed since then.

Working for a music publisher, Guest happens to reunite with Seto, who has become a famous composer, through work.

But the kind teacher from that day is gone.

He has changed into a man as cold as a stranger, keeping everyone at a distance.

"That's in the past. Forget it."

Even being told that, Guest's first love cannot end.

Then, the teacher's younger brother appears before her.

Haruka, who is the polar opposite of his brother—friendly and kind—knew that his brother was still suffering, unable to forget Guest.

While supporting Guest, he himself falls truly in love with her.

The first love that was supposed to have ended ten years ago begins to move again, entangling the two brothers.