This is mostly a prequel showing MC's past and first love(Momo) who betrayed him in the most NTR fashion (drunk, rape and addicted to dick) and shows the reasons why he got a cuckold fetish. He found out of Momo's cheating through a classmate, confronted her properly and she admitted, he didn't break up with her and they promised each other to overcome that together yet she still cheated more and both ended up breaking up. To the yarichin Momo was just a sexfriend with no more value than being an easy lay. Years later during his coming of age he met his future wife Nozomi who reminds him of Momo. Time jumps to the present and once again Takumi is watching the appointed cuck session.
In pages 8-9 there's a cameo of Nozomi getting fucked by Eiji who was her fuck buddy back then.