Koichi Takakura, a former profiler who resigned after an injury, and his wife Yasuko relocate near his new teaching post in criminal psychology. To sound friendly, Yasuko reaches out to their two neighbors. The first neighbor, Mrs. Tanaka, and her bed-ridden mother offer a cold, distant reception and rebuff any attempt at friendship. The couple’s other neighbor, who sits between the Takakuras and the Tanakas, initially remains anonymous. When Yasuko leaves a bag of chocolates at this middle neighbor’s gate, the man who eventually introduces himself as Masayuki Nishino appears and makes a strangely uneasy impression on her.
At Koichi’s university, a cold case catches his attention: three members of a family vanished, leaving only a highly unreliable witness, Saki Honda. A former colleague, Nogami, asks Koichi to help pursue the inquiry. At the crime scene, Koichi and Nogami try to speak with Saki, but she struggles to recall the events surrounding her family’s disappearance. Back home, Yasuko encounters Nishino again; he tells her he lives with his daughter Mio and his wife, but then coldly dismisses her when she expresses a desire to meet them.
As Koichi and Nogami dig deeper, they learn that Saki’s memories are inconsistent and that the Honda family may have encountered someone watching from the Mizuta house, the next-door property owned by a man named Mr. Mizuta. While investigating the Mizuta residence, Nogami uncovers five decomposed bodies wrapped in plastic. The tension heightens when Mio helps Yasuko search for their dog, Max, and Nishino makes a pass at Yasuko, further raising Koichi and Yasuko’s alarm about the stranger who seems to control the social space around them.
The situation grows more disturbing when Mio reveals that Nishino is not her father, but a stranger who has insinuated himself into their lives. Nishino’s presence casts doubt on every ordinary exchange, and Mio’s fear intensifies as Nishino returns home. Yasuko begins to show signs of instability, alternating between sickness and emotional outbursts, deepening Koichi’s suspicion. He asks Nogami to look more closely into Nishino, and the two men eventually get a look inside Nishino’s house. Nogami goes down a long corridor and discovers a large metal door; just as Koichi rushes back, the Tanaka house explodes, with Nishino casually watching television as chaos erupts nearby.
The law enforcement investigation confirms three bodies inside the wreckage: Tanaka, her mother, and Nogami. Koichi revisits Saki in hopes of a memory that might link Nishino to the disappearances, but she does not recall him. It becomes clear that Nishino is an impostor who manipulates Mio and her mother through drugs, and that he may have killed Mio’s real father, forcing Mio to dispose of his body alongside the others from Mizuta’s residence. Mio’s mother is kept under a calm-inducing regimen until Mio tampers with her dose, at which point the mother attacks Nishino but is subdued. Nishino then shoots the mother with Nogami’s gun in front of Mio, demonstrating the brutal reach of his control.
Nishino drags Yasuko into his chamber, forcing her to witness the body and to cooperate with Mio in disposing of it. Koichi returns home to find Mio barging in, and Nishino uses Yasuko’s house key to keep the family under his grip. Koichi assaults Nishino, but the police arrive and detain him, even as Nishino escapes custody and heads to his own house.
lieutenant Tanimoto tracks Nishino and discovers the hidden chamber, but Nishino incapacitates him. Koichi follows into the chamber and finds Yasuko and the injured Tanimoto, only to be confronted by Nishino, who holds Yasuko at gunpoint. Nishino explains that he has effectively brainwashed Yasuko and Mio with drugs, and that Yasuko is now his. Koichi realizes Nishino ruthlessly moves from family to family, bending them to his will until they turn on each other. He urges Yasuko to see the truth, but Nishino injects Koichi with a drug, and the two families—Koichi, Yasuko, Mio, Nishino, and Mio’s dog Max—are forced into motion.
The group arrives at an abandoned building, where Nishino scouts for a new “home.” He deems Max an unnecessary burden and tries to push Koichi to kill the dog. In a tense confrontation, Koichi seizes control of the moment and declares, “this is where you fall,” then turns and shoots Nishino. Free from Nishino’s brainwashing, Mio celebrates with Max, and Yasuko, finally understanding the ordeal they endured, collapses into Koichi’s arms, grateful for his protection and ready to rebuild their life together.
