For three weeks, Detective Jeon Jungkook has been chasing a serial killer known as the Key Killer—a murderer who leaves an antique key beside every victim. With the investigation at a standstill and pressure mounting, Chief Kim Namjoon makes a controversial decision: recruit convicted murderer Park Jimin as a consultant. Jimin only has three years left on his sentence for killing the man responsible for his sister’s death. In exchange for helping solve the case, he is offered his freedom. Armed with an extraordinary intellect and an unmatched ability to read people, Jimin quickly notices connections the police have overlooked. But the deeper he and Jungkook dig into the Key Killer’s twisted game, the more they realize the murders are part of something far larger than anyone imagined. Forced to work together despite their mutual distrust, Detective Jeon Jungkook and consultant Park Jimin must follow a trail of secrets, lies, and carefully placed clues before another victim falls. Because the Key Killer isn’t just committing murders. He’s sending invitations.