Will a killer-turned-veterinarian and a detective find common ground? 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. Do Hyun Jin was once a ballerina, but now she is a detective working in Seoul, who graduated from police academy as the most excelled. Although she looks cold and stern on the outside, she is, in fact, a very sympathetic individual. Hyun Jin trusts her instincts and isn't afraid to rely on the things around her to make a decision to the best of her abilities. She has suffered trauma but still keeps pushing forward. She starts to chase after Kim Soo Hyun as she believes he is a serial killer. What happens when both get to know that the connection between them has started years ago?