A killer must be prepared to put on the perfect act for their assassination to be successful. They must be exceptionally meticulous when erasing any trace of evidence, and they must absolutely never be caught. These are some of the simple rules that Joo Taeman has followed for 14 years, never allowing his emotions to interfere. Meanwhile, the famous actor Kang Dahyeok, nicknamed the "Heart Assassin" due to his attractive looks and dreamy eyes, is a huge fan of gritty film noirs. He has always dreamt of starring in something with an edge, but he keeps getting stuck with swoony romantic dramas. After witnessing Taeman's ability to effortlessly switch from a scowl to a smile, Dahyeok is determined to befriend him and learn the trick. However, Taeman falls for Dahyeok at first sight, facing a dangerous challenge: how to separate his high-risk profession and this deadly, killer crush! [Written by MAL Rewrite]