Basically, the story takes place in Life 2.
In Life 1, Jarn and Ferir meet after Ferir saves Jarn during his childhood. Ferir is obsessed with/loves Jarn and wants to spend his life with him, but Jarn is oblivious to this. The two share an intimate and comforting relationship. Jarn meets the Duke by chance; after a while, the two “fall in love.” The Duke is abusive toward Jarn, and Jarn confuses the abuse with passion. The result: an abusive and obsessive relationship that only gets worse because Duke is completely insane and takes it out on Jarn. Duke locks Jarn up; Jarn becomes depressed by the situation—plus the situation with his mother and aunt (he’s looking for his family)—and wishes to go back in time. Ferir, who is always there to comfort Jarn, grants this wish from time to time and goes back in time because he wants Jarn to be happy and live a life with him.
Life 2: Jarn is looking for his aunt, which leads him to the Duke’s mansion because a woman said the Duke has some connection to his aunt and family. He looks for a job at the Duke’s mansion; the Duke isn’t as crazy because he grew up seeing the spirit of Jarn as a child (influenced by memories from Life 1 that he doesn’t remember), which made him more decent. The Duke isn’t as abusive anymore. At the mansion, he’s given the job of taking care of the monster in the dungeon. Jarn goes to the dungeon and finds a monster in the form of the Duke, who is later revealed to be Ferir, but Jarn doesn’t remember Ferir. Later in the story, we see Ferir getting closer to Jarn and trying to make him remember his first life; Jarn tries hard to remember, but every time he remembers a little, he faints or gets sick.
(Tried to simplify the story. Sorry if I left out a lot of information, hope it helps)

Bruh, ima be so real right now, I'm a dumb bitch with this plot. Like I get it, but also kinda don't. Can someone spell it out for me, please.
Thank you! ┗( T﹏T )┛