University lecturer Shunsuke Goto appears to have built a happy family with his beautiful wife Akemi, a painter from a wealthy family, and their two children, daughter Moe and son Ken. But this happiness is a fragile facade. Shunsuke is tortured by the suspicion that Akemi is being unfaithful, and he is haunted by disturbing dreams in which he has sexual relations with his own daughter Moe. His mental state begins to fray under the strain of these daytime suspicions and nightly nightmares.

One day, Shunsuke returns home to find Moe's room ransacked and her in tears. Convinced that Akemi is responsible, he vows to protect his family at all costs. However, the destruction was actually orchestrated by Moe herself, who harbors an abnormally intense, romantic obsession with her father. As Shunsuke's suspicions drive a wedge between him and Akemi, Moe positions herself as her father's confidante. She subtly manipulates events, escalating her hold over him and eliminating anyone she perceives as a threat. Ken, the younger son who bears physical and emotional scars from Akemi's past violent instability, grows suspicious of Moe. When he discovers that she has been secretly using Akemi's phone to contact Akemi's former lover Yabuki, Moe pushes him down a flight of stairs, leaving him hospitalized with potential memory loss.

As the family disintegrates, Shunsuke learns that Moe may not be his biological daughter, a revelation from nineteen years ago exposed by a new pregnancy of Akemi's. This knowledge erodes Shunsuke's remaining boundaries. Moe openly offers to act as his wife, and they visit the site of his old proposal to Akemi. Meanwhile, Ken, now aware that Moe tried to kill him, hires a detective to investigate her. Moe, in turn, expertly plays the roles of victim and caregiver. She also draws the attention of a predatory senior lecturer named Mitsuhashi, who sees her as easy prey. The situation intensifies as other characters become entangled: a mysterious figure who calls Moe his "fateful person" appears, and an admirer named Kamiya goes so far as to bomb the detective agency for her sake. The story escalates to a climactic confrontation where Ken plans to kidnap his own father to save the family, and a vengeful party secretly swears retribution against Moe. In the end, the Goto family, twisted by love, lust, and hatred, reaches a final, dark conclusion as each person connected to the "monster" makes their last move in the shadows.