Why?

Yeonjun and Beomgyu are enemies, they don't usually fight and they almost always ignore each other

One day, everything changes, Beomgyu doesn't show up and no one seems to remember him from one day to the next, Yeonjun has nightmares of what could have happened to him and when he wakes up, he wakes up a week before Beomgyu completely disappeared, he has traveled to the past and he will take advantage of it