Chapter 1: Yu
Yu Honda, a young teenage Catholic is attempting to live his life in a faithful and orderly manner. Before his mother dies, she makes him promise that he will marry a woman as beautiful as The Virgin Mary. His father, Tetsu, becomes a devout Catholic priest following the death of Yu's mother, and operates his own church. As he grows up, Yu comes to realise that only marrying Mary will suit him, and goes on his search for Mary. A woman named Kaori becomes obsessed with Tetsu, and forces her love on him, which he eventually reciprocates. Together, the three move into a new house where their relationship is kept a secret. Tired of this secrecy, Kaori leaves the house, which leaves Tetsu broken and depressed. He makes Yu come to confessional every day, and demands to know his sins. Yu, unable to think of any sins he might have committed, lies, but is caught. He decides to deliberately commit sins and confess them to his father.
Tetsu becomes displeased with this, and moves to his church, almost disowning Yu as his son. Yu falls in with a gang of three delinquent boys named Takahiro, Yuji and Senpai, who teach him to steal, fight and live on the streets, amused by his need to commit sins only to confess them to his father. They suggest he try something more provocative, which a Catholic priest would not tolerate, and take him to a man named Llyod, who specialises in a "kung fu" technique of taking voyeuristic "panty shot" photos of girls in public. After great training, Yu becomes an expert himself at taking "panty shot" photos, but unlike others, does not do it for perverted reasons, but simply out of a need to commit sins. When he confesses his activities to his father, Tetsu beats him as a father would beat his son, which pleases Yu. He refuses to take Yu's confession, and throws him out of the church, where Yu piously prays to God even in the rain.
A gang of three girls, led by Koike pass him and he photographs them, but is caught. Koike however, lets him go, amused about his situation. Yu's friends see this and are greatly inspired by Yu, whom they approach to be trained themselves. Yu obliges, and the four operate as a ring of voyeur photographers, comparing each others photos and rating them. On one such day, Yu bets that if Yuji's photo is better than his, he will wear female clothes, kiss a girl he likes and profess his love in a female voice. Yu loses the bet, and goes out wearing the outfit worn by Meiko Kaji in Female Convict #701: Scorpion, and refers to himself as Sasori (Scorpion).
He finds a girl being attacked by a large group of thugs, while she stands off against them. Taking this to be his chance, Yu attacks the group, fighting side-by-side with the harassed girl, whom he sees as the Mary he had promised to marry. The title card is shown at this point.
Chapter 2: Koike
Koike, who performs many fraudulent and illegal activities such as drug-dealing, works for a New Religious Movement called the Zero Church, of which she is the regional leader. The daughter of a man seen by society as a model Christian, she was frequently abused by him as a child, physically and sexually. As she grows up, she comes to resent all sex and violently attacks men. One day, she finds that her father has had a stroke, but with a full erection. She takes her revenge by snapping his penis and then chopping it off.
Extremely happy after doing so, she ends up meeting the Director of the Zero Church, who inducts her in their ranks and tells her that she has the unique ability to seek out the "original sin"s of others (which she explains is how she came to discover Yu). Unbeknownst to Yu however, she continues to observe and film him, electronically eavesdropping on his conversations with his father and in a board meeting of the Zero Church, proposes a plan to convert Yu's entire family.
Chapter 3: Yoko
Yoko grew up with an abusive father who often brought women home and neglected her. Growing up, Yoko becomes fatalistic and comes to hate the very idea of a happy family. She develops a hatred for men, but her life is changed when she meets Kaori, whom her father brings home one day. Kaori and Yoko come close to the point where Yoko accepts Jesus, while being treated to every whim and fancy by her new foster mother Kaori.
Kaori however, returns to Tetsu's church and beings to force her love on him, which he resists. Obsessed with him, Kaori rams her car into his, crashing it until she can finally achieve him again. Tetsu returns home with Kaori, much to Yu's surprise, and declares that he will be giving up his priesthood in order to get married to Kaori. When Kaori breaks the news to Yoko, she gets angry and walks away.
Meanwhile, Koike, who had been observing Yoko as well as Yu, organises the group of henchmen who harass her before Yu, prompting his intervention in the fight.
Chapter 4: Sasori
Fighting as the enigmatic Sasori, Yu inspires Yoko and the henchmen are dealt with. As per the bet, Yu kisses her after the fight and tells her that he loves her before departing. After this event, Yu realises that this is the first time he has had an erection to a girl - confirming that the girl, Yoko is his Mary.
Yu becomes infatuated with Yoko, while Yoko comes to believe that she is a lesbian and becomes infatuated with Sasori. Yu loses interest in his "panty shot" photography, and finds that Yoko will be joining his class as a new student. As Yoko has no interest in Yu, he finds himself unable to approach her directly. When dressed up as Sasori however, Yoko lets her guard down. Yu gives her his number, and finds later that night that Tetsu will be marrying Kaori once he is relieved of his priesthood by the Vatican. Yoko is repulsed when she learns that Yu will be her brother, and calls Sasori. Yu, as Sasori, advises Yoko to be more understanding of him and to understand that perverts have a reason for being the way they are. Sasori ultimately makes Yoko promise that she will treat Yu more affectionately, and as a brother.
Yoko does as told, and this sudden improvement in their relations pleases Yu, who still finds himself unable to reveal to her that he is Sasori. The next day however, Koike joins Yu's class as a new student. As per Koike's instructions, henchmen attack the class seeking Sasori. Yu is pushed aside by Koike, who dispatches the thugs and declares that she is Sasori, before Yoko. Yoko is overjoyed to learn that Sasori is in her class, and the two start to have close sexual relations, much to Yu's chagrin. Koike repeatedly taunts Yu, who is infuriated with her designs.
Meanwhile, Tetsu's faith in Catholicism starts to waver as he is chastised by his superiors for wanting to give up priesthood in favour of romance. He breaks a long-standing family tradition by not saying grace before dinner. Koike embeds herself in Yu's family, coming to help Yoko with her homework and often for sleepovers, in which she'd have sex with Yoko. She also comes to be the family's spiritual guide and counsels Tetsu and Kaori. When Yu learns that Koike slept with Yoko in his Sasori outfit and has discovered his stash of photographs, he knocks her out cold and decides to reveal the truth about Sasori to Yoko.
Yoko however is repulsed by this, and runs away, leaving Yu helpless and angry. The next day, Koike spreads videos and photos of Yu's perverted photos in the school, leading to Yu's expulsion. Yu's family becomes estranged from him, and Tetsu beats him. With Yoko avoiding him, Yu runs away from home, running wildly in the streets screaming Yoko's name. He is eventually found by his friends Senpai, Yuji and Takahiro, and returns to takes refuge in them and recounts his tale.
Sneaking into school, Yu learns that both Yoko and Koike have stopped attending school. He finds his home abandoned, and that Tetsu has been missing from Church. Koike managed to convince the family that it was the parents' immoral and sinful behaviour that caused Yu to become a perverted sinner, and inducts Yoko, Tetsu and Kaori into the Zero Church, where they are placed under brainwashing sessions.
Yu receives a call from Koike, taunting his situation and advising him to join a porn video company called Bukkake-sha, where he is accepted. The company president is pleased with his skills and assigns him a job filming adult videos in the Tosatsu genre, which is Yu's specialty. Yu is told by Koike's sidekicks that he will continue to work in Bukkake-sha if he wants to see Yoko again. The videos become highly successful, as Yu is motivated by the promise of seeing Yoko again. The company president is impressed with Yu and offers him an active role in adult videos, which Yu fervently denies.
Yu is instead placed as a performer in a "weird show" for perverts, wherein he is introduced as the immoral son of a Catholic priest, who "forgives" perverts for their perverted thoughts that they confess to him. He eventually meets Koike's sidekicks again, who show him a video of a brainwashed Yoko. Furthermore, he finds Yoko in a TV report on an attempted suicide in the Zero Church, and he takes researches means to recover Yoko from the Zero Church, but is largely unsuccessful.
One day, Yu's friends organise a plan in which Yoko is abducted and taken to an empty beach, tied up and left alone with Yu till she breaks. Yoko refuses to give in, repeatedly calling him a pervert, and a few days into her abduction, tricks Yu into releasing her. Yu catches her and tries to convince her to leave Zero Church, calling it a sham, declaring that even if he is a perverted sinner, he at least has the dignity of being one. Yoko recites the entirety of Corinthans 13, and refuses to give into what she perceives is Yu's perverted love.
The next day, Koike and the director of the Zero Church arrive at the beach, having caught Yu's friends. Yu is beaten up and Koike orders Yoko to cut off Yu's penis, while Yu hesitates. Eventually, the director intervenes and invites Yu to join Zero Church, under the condition he strive to get rid of his pervert nature, and prove it by not having an erection while facing Yoko. Yu joins the Church and manages to not have an erection while facing Yoko, and is taken as a dedicated member of the Church.
Pretending to be a dedicated member to the cultlike organisation, he discovers Yoko's location and procures dynamite, a katana, a knife and the Sasori outfit, with the help of his friends. He raids the Zero Church facility where Yoko is being kept and brainwashed, and kills the Zero Church's director. Dressing up as Sasori, he arrives before Yoko alongwith his family and Koike. Restrained by Tetsu and Kaori, and with Yoko retreating away from him, he uses the dynamite to blow up a lower floor of the facility, forcing police intervention.
When he starts to attack Koike, Yoko starts to strangle him to death, causing him to bleed from his eye, making it look like he's crying blood. Yu breaks free, but has a nervous breakdown and becomes mentally affected. Koike is amused by Yu's breaking down, and says that he has become the same as her and kills herself with the katana. All the while, Yoko cries out of fear and stress.
Once the cult's brainwashing activities come to light, it is closed down. Tetsu and Kaori join a victim support group and appear to return to normal. Yoko goes to live with her relatives, where she uneasily settles into ordinary family life again. She comes to profusely miss Yu, watching the Tosatsu adult videos he made. Eventually, she breaks down on remembering how he cried blood when she tried to strangle him. She berates herself for not having known anything then, and for never having understood Yu.
Yu is placed in an mental institution where he has developed a complex in which he is Sasori, and he attacks invisible foes with his katana. Yoko comes to meet him and finds that he does not recognise her, and greets her as Sasori, only remembering that he once saved a girl like her. Yoko presses on him and insists that he is her brother and that he opened her eyes and rescued her from Zero Church. She drives the doctors away and is forcibly taken away by the police, while Yu remains in shock. Shortly thereafter, however, he sees a reflection of himself from below in which he sees his erection, and realises that he is a man.
Having flashbacks of his childhood, and the fight in which he first met Yoko, he regains his memories and starts running frantically out of the institution, meeting up with Yoko as she is being taken away by the police. At long last, the two are finally united.