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'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.