A double homicide occurred in a quiet residential area.

The victims were Yujiro Takanashi and his wife, Masami.

The two were found collapsed by the window of their home, as if huddling together. The cause of death was blood loss from stab wounds. The room was violently ransacked, and no murder weapon was found. The police began their investigation considering the possibility of a robbery-homicide.

The bodies were discovered by their son, Seiji Takanashi.

However, at the time of death estimated by the police, Seiji was attending a dinner party with friends. With multiple witness testimonies and security camera footage remaining, Seiji had a perfect alibi and was removed from the suspect list.

Seiji requests an investigation from the detective, Guest, asking to find the culprit who killed his parents.

Below are the materials provided to Guest by the police.

However, shortly after the investigation began, a second and third murder occurred.

There are no commonalities in the victims' ages, genders, or occupations. They had no acquaintance with each other, and no connection to the Takanashi couple was found.

Nevertheless, the police concluded from the characteristics of the wounds that it was a serial murder by the same perpetrator.

Furthermore, the new victims were all found near windows.

It was as if the culprit was trying to prevent the case from ending—.

The assistant who admires Guest, Minato. The calm detective cooperating in the investigation, Tomoharu. The friend who stays by Seiji's side out of worry, Kei. And the bereaved family member, Seiji.

The four people involved in the case each harbor secrets they can tell no one.

What is the lie hidden behind the perfect alibi? What is the reason for placing the victims by the window?

Whom will you trust, and whom will you doubt? Who on earth is the serial killer—?