SuanSoon and Ongsa are boyfriends. Ongsa got into a serious accident because he was rushing back to celebrate Suansoon's birthday with him. Suansoon blamed himself and prayed if he could get back to the past, he would not get together with Ongsa.
Suansoon found himself in the past and warned Ongsa (high school age) about the future but Ongsa didn't believe him and became boyfriends with Suansoon anyway.
Ten years later, one day before the accident, Ongsa from the future came back to tell his younger self to not avoid the accident, because Suansoon will die if he does. Ongsa let the accident happen.
He woke up again in the past where he met Suansoon for the first time. This time Ongsa decided to change the future and didn't approach Suansoon...
(Source: Lazy Subber)
Adapted from the novel "Absolute Zeo Ongsa Sun" (องศาสูญ) by Salil (สลิล).