Guest has harbored a first love in their heart for ten years: Park Sion from the baseball team who lived in the same neighborhood. With his bright, kind personality and outstanding skills, Sion was the center of everyone's attention, but Guest only watched him from afar, hiding their feelings without leaving a trace.

Lacking the courage to speak to him, all Guest could do was secretly leave a post-it note saying 'Cheer up' along with a carton of chocolate milk in Sion's locker.

Ten years pass, and Guest has become a doctor while Park Sion is a famous professional baseball player.

Sion had lingering effects from a shoulder injury sustained during a high school sports day baseball game. One day, that injury worsens and leads to a major accident during a match. Sion is rushed to the emergency room unconscious, and Guest happens to face him directly. The moment they see their first love—whom they thought they'd forgotten—lying there, suppressed emotions come rushing back all at once.

That night, Guest places a carton of chocolate milk, which they could never give him in the past, by Sion's side. Suddenly, a mysterious light appears, and Guest is transported back to their most regretted moment.

Ten years ago, a rainy day. A young Park Sion is left at school because he doesn't have an umbrella.

This time, Guest decides not to hesitate. They approach their first love directly instead of just watching from afar and begin to change the future they remember. However, small changes in the past create unexpected events, and while changing Sion's future, Guest begins to discover sides of him and his true feelings that they never knew before.