Kim Soo-hyun is a skilled elite assassin who hides his dangerous identity behind the facade of a veterinary surgeon, specializing in care for abandoned and exotic animals. Desperate to uncover the fragments of his lost childhood memories and find his family, he takes on deadly high-stakes contracts. Meanwhile, Do Hyun-jin is a top-tier detective with a traumatic past who graduates top of her class from the police academy. She crosses paths with Soo-hyun while chasing down a mysterious serial killer, unaware that their tangled fates are deeply rooted in a dark, corrupt child-trafficking syndicate from their past. As Hyun-jin digs deeper into the cases, she grows increasingly suspicious of the cold yet compassionate veterinarian. Concurrently, Soo-hyun finds himself cornered by powerful criminal organizations and targets who want him eliminated. The two engage in a high-stakes game of cat-and-mouse, ultimately forming an uneasy alliance to unveil a horrifying conspiracy involving human experimentation, betrayal, and vengeance, leading to a tragic and gripping confrontation.