Onotora Shu does not know that his foster father is a legendary professional killer. However, his daily life dramatically changes after someone murders this professional killer. Onotora decides to take over the family business and become a hitman in order to avenge his foster father. He is well prepared when he approaches Narumiya Mitsuki who is his assassination target. Despite his physical abilities as a hitman, he is not able to kill her because of his terrible timing, no matter how he tries. On the contrary, he ends up protecting her as danger approaches. The two of them, who should not fall in love, have an unlikely romance.