Season 1

After getting an officer suspended from service, Ryu Su-yeol gets a step closer to a promotion. Oh Gyeong-tae looks for a young girl's missing mom.

Su-yeol retraces Gyeong-tae's steps while tracking down the man in the helmet. Lee Hui-gyeom goes after a necklace that may hold crucial evidence.

Su-yeol falls from Do Yu-gon's good graces. While searching for the necklace at home, Su-yeol finds a collection of objects that are not his.

After ensuring Gyeong-tae's safety, Su-yeol pulls a dirty move. With Kim Gye-sik back on the force, the narcotics unit tracks down a street drug.

Su-yeol searches high and low to prove Hui-gyeom's innocence. But the deeper he dives into the case, the closer he comes to falling into a trap.

Su-yeol reopens a closed case. He teams up with K to investigate quietly and uncovers the opponent's weakness. Meanwhile, the body count grows.

Su-yeol and Hui-gyeom shift focus to the head of the drug cartel. Ms. Yong threatens the police to get the seized assets back in her hands.

Su-yeol learns that K is closely linked to his childhood trauma. A girl shows up at the police station, delirious and covered in someone else's blood.

When bits of his memory return, Su-yeol reexamines the murder of his father. Finding similar cases, he learns one person was involved in all of them.

As the identity of X comes to light, his deranged schemes pick up pace. He comes between Su-yeol and K by revealing K's murderous instincts.

Su-yeol visits Shin Ju-hyeok to seek treatment, but unleashes a very angry K instead. A victim opens up after an attempted suicide.

Su-yeol's hunt for the truth endangers those around him. He has a final face-off with the crazed killer at the place where it all began.