Main character Inu ("dog" in Japanese) was left by his father when he was very young. He was adopted by a sadist couple who gave him daily beatings. One day he suddenly starts to read the thoughts of the old couple and learns that he can be killed any second, so he dares to rebel. He kills both of them and runs away.
From that point and on, he lives the life of a vagabond, stealing food and money to survive - and often raping women as his only enjoyment in life. One day he follows a girl he suspects of having witnessed a rape he committed last night. Following the girl, he eventually reaches a big mansion where he sees the girl together with another girl, entering the mansion. At that point, Inu prepares to leave, when he suddenly senses someone behind him! Some boy asks about purpose and easily defeats protagonist. When Inu regains consciousness, he’s in a mansion room looking much like an office - with the boy who beat him next to him. After some talking between the two of them, a look in a mirror, the showing of a secret diary and secret room, it becomes clear that the two of them are brothers and twins! Boy Kagura Yoshiaki then offers Inu to join the family, but Inu convinces Yoshiaki to let go of that thought, since that would mean that they'd have to tell the rest of the family of some of their father's... more dark secrets, which might break the family or at least, not make them very happy. However, he lets himself be convinced into staying at the mansion, living mainly in the secret room plus the office he was brought into in the first place, watching Yoshiaki and his family from afar. Through his watching, he learns to know Yoshiaki's family and those closest to him.
[Edited from Spectator_Beholder review]