When Hiroyuki was younger, he and some of his friends repeatedly sexually assaulted a male classmate.
Hiroyuki’s father found out about this. To cover it up, he imprisoned the classmate and his older sister, revealed to be Kako. During the imprisonment, he frequently sexually abuses them both.
One day, Kako’s brother dies from the father’s abuse. In a rage, Kako brutally mutilates and kills the father with a key. Kako then sees pictures of Hiroyuki assaulting her brother on the father's phone, realising that their imprisonment stemmed from what Hiroyuki did.
Hiroyuki suppresses these memories and goes on to live a normal life until Kako meets him again in chapter 1.
By the time Hiroyuki remembers his past, his life is already completely ruined. In the penultimate chapter, after revealing the truth, Kako commits suicide by mutilating her own face with her key then slitting her throat.
The story ends with Hiroyuki living with the guilt of the horrific past and the destruction of his present life. Everywhere he goes, he sees Kako’s mutilated face.
There is some semblance of a happy ending as he's shown to still have a bond with his daughter, who is now a few years older in the final chapter. However, he sees his own hands as permanently covered in blood, even when holding his daughter's hand.