A man named "M" invites seven people to a murder game party. Among the eight in the room, one will become a victim, and another the culprit.

Recognized for their performance, the detectives report to Jeju Island, where their first client asks them to participate in a treasure hunt.

While enjoying their break on Jeju Island, the detectives get a shocking call from K, who says Kwang-soo is a prime murder suspect and is on the run.

Jong-min comes across a diary of a dead teen girl who was possibly murdered. To find the truth, he gathers the detectives at the school she attended.

Jong-min encounters a child who gives him the diary of a teen girl. He finds out that the girl had recently died at Songdo Central Park under mysterious circumstances and receives a call from K, who tells him to gather the detectives at the high school where she attended to look into her death. During the investigation, the detectives discover that her friends have been receiving text messages from her even after she died.

An ex-criminal, whom Jae-wook had arrested during his days as an agent, tells him that a woman was murdered in his hometown, a small island near Incheon. When the detectives go to the island, they discover that the residents believe that a vampire lives in the island and kills a woman whenever anyone tries to investigate on the murders happening in the village. Meanwhile, K tells them that one of the doctors involved in Project D was from the same island.

The detectives attend a magic show rehearsal to support Se-hun, who would be opening the show with a dance performance. During the rehearsal, the magician mysteriously disappears while performing one of his tricks, which sends the crew in a panic as the show would be happening in a few hours. As a result, the director asks the detectives to find him before the show begins.

A man claiming to be K's old friend challenges the detectives to solve a 20-year-old case through a series of tests in order to measure their skills in criminal investigation. Each test has a different goal, and the first detective to complete a test is rewarded with a large sum of money. The man also promises to provide the passcode for the box that the detectives had obtained in Jeju Island, which contains valuable information about Project D. Outside the mission, Se-jeong is seen leaving flowers for M at a columbarium.

Yeon-seok turns himself in and requests that K's detectives should be the one to transport him to prison. Although this confuses the detectives, they accept his request in exchange for information about Project D and his involvement with it. While on their way to prison, Jae-suk receives a phone call from an unknown man demanding Yeon-seok to be brought to him, or else, a bomb that he planted onto a stranger's bag will be detonated.

The detectives receive a scheduled message from K that was written three months ago, during the planning of the murder game party. He tells them that a final test for them is waiting at a safe house in Incheon. However, they also receive a message from a familiar voice saying that he has kidnapped K.