To ensure an assassination succeeds, a killer must be ready to perform the perfect act. They need to be exceptionally meticulous in erasing any trace of evidence and must absolutely never be caught. These are among the simple rules Joo Taeman has followed for 14 years, never letting his emotions get in the way. Meanwhile, the famous actor Kang Dahyeok, nicknamed the "Heart Assassin" because of his attractive looks and dreamy eyes, is a huge fan of gritty film noirs. He has always dreamed 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 from this deadly, killer crush!