The main story is about 3 guys. Mitsuru is an uke. He used to go out with a guy for 5 years - it wasn't official. One day, Mitsuru's partner at the time, Kouchi showes up after 2 or 3 months of not seeing each other with a wedding ring on his finger. Turns out he was married now but wanted to continue to have Mitsuru as a sex friend. Mitsuru got a little pissed so he stopped contacting him and moved out. That time he finds a guy Shougo who is a bar owner and who had a crush on him for a long time. They end up dating. But 1 day Kouchi showes up and messes up Mitsuru's head. They ended up sleeping together and Mitsuru stayed with him for a while (but his mind was broken... basically... cause his ex turned all psycho and started to be rough in bed.) Eventually his ex got the message and left Mitsuru alone. Mitsuru went for Shougo in the end.
Chapters 4 and 5 of volume 1 is the original couple's flashback when they were students. They were childhood friends who just started to go out. They had some misunderstandings but they made up in the end. Then it turns in the present.
Doronuma