For centuries, "Poison Masters" have been renowned for their killing prowess, and the most prominent among them is the Gero clan. Hikaru Gero is the heir to this clan, having spent his entire life in the dark underworld. Love and marriage are two things he has no connection to. But the twist comes when the elders of the family decide that if Hikaru doesn't marry, his sister will be forced to bear him a savage heir. To save his sister from this predicament, Hikaru must marry quickly. During this time, he meets Mei Kinosaki, an expert at deceiving people under the guise of marriage. Interestingly, Mei is Hikaru's next target. But instead of killing her, Hikaru makes a strange offer—he asks Mei to help him get married. On one side, the world's most dangerous assassin, and on the other, a feisty thug; together, they embark on the most difficult mission of "marriage."