Joi, Shu's foster father gets shot and later finds out that Joi was a hitman for an agency called SOS from Joi's friend/boss Shiori. Shu takes it upon himself to not let Joi die in vain and takes his place to assassinate the woman who shot him, who is a famous manga writer, Narumiya Mitsuki. Shu later shows up at an event Mitsuki was attending to assassinate her but ended up saving her life twice from another hitman, Death Prince. (Source: Linguini68plus1 at MyDramaList)