Kim Soo Hyun is a veterinarian who is also a professional killer. Many individuals and organizations hire him for large sums of money. Nobody in the public knows anything about him as a killer, including his age, name, and race. Animals are his only companions, but deep inside, he constantly yearns for love. He struggles to develop and maintain any pure relationships around him. One day, a client asks him to kill in exchange for information regarding his mysterious past and a conspiracy involving his birth.