Once upon a time, there was happiness, there was love. But with only one little mistake came pain and sadness, destruction of friendship and hate.

Kyungsoo decided to bury his past with the ones he lost. He wouldn't talk about it with anybody. He even hid his pain from his bestfriend, Sunji.

But when the past comes back, when everything he thought would never happen really happens, Kyungsoo knows he made a big mistake, again. He thought he was doing a good thing by hiding the truth to the person he loved, but actually, it only made everything worse.

This is not the story of his present, but the story of his past. The events that haunted him, Jong In and Baekhyun for years. The reason for that second mistake. The reason for their changes. The reason for everything.