Under the leadership of Mikoto, the UDI team (Unnatural Death Investigation Unit) once again faced a bizarre case.This time, they received a commission on a suspected mass suicide by burning charcoal.However, as the investigation deepens, Mikoto firmly believes that this is not a simple suicide case, but a carefully planned murder.

The case took place on a cold winter night when a group of young people were burning charcoal to keep warm in a remote hot spring villa.However, when rescuers arrived at the scene, they found one of the girls dead.After a preliminary autopsy, doctors initially determined that the cause of death was carbon monoxide poisoning.However, Mikoto discovered something fishy.She discovered that the girl's cause of death was not carbon monoxide poisoning, but freezing to death.What's even more shocking is that there was a note calling for help left in the girl's stomach.

This note convinced Mikoto that this was a murder case.Her point of view was blocked by the director.The director believes that this is just an unfortunate suicide case and does not require too much investigation.But Mikoto insists on unraveling the message left by the girl and uncovering the truth.She led the team members and began a difficult investigative journey, ignoring the obstruction of the director.

On the way to the hot spring villa, Mikoto and team member Rokuro discovered a surprising fact.It turns out that the owner of this villa is the girl's father.And he mysteriously disappeared on the eve of the crime.This discovery made Mikoto even more convinced that this case was not a simple suicide, but a carefully planned murder.

As the investigation deepens, Mikoto and Rokuro gradually discover more clues.They found that the charcoal-burning equipment in the villa had been deliberately damaged, causing carbon monoxide to leak.The girl's father is the mastermind behind all this.In order to cover up his crime, he deliberately locked his daughter and other young people in the villa in an attempt to create the illusion of suicide.

Just as Mikoto and Rokuro were about to reveal the truth, they encountered a life-and-death crisis.After the girl's father learned that his crime was about to be exposed, he decided to kill Mikoto and Rokuro.He set a trap to kill them.