A genius police officer, Cheon Jae-in, and an aspiring officer, Kim Dan, use individual methods to investigate as a large number of people die within a religious cult.

Jae-in feels guilty of his sister Su-in's death and lives among the homeless to find clues of the murderer. He runs into Dan and asks her about a case that happened 2 years ago. Meanwhile, Dan and her team attend a memorial for a dead heiress.

Jae-in and Dan cooperate to investigate as a serial killer reappears 2 years after its last killing. The two find evidence while working together. Meanwhile, the supposedly dead heiress breaks her silence.

The serial killer who murdered Jae-in's sister dies at the hands of the police and Jae-in searches for other ways to continue the investigation in order to find out who ordered the killing.

A body is found on the beach of the island Jae-in has visited. The local residents are identified to be possible suspects. Meanwhile, Dan recalls faint memories of her childhood.

A deadly secret is revealed on the island making the local residents become aggravated and aggressive. Meanwhile, Jae-in and Dan find a survivor of the case 24 years ago.