For centuries, the Poison Masters have refined the art of assassination. Among them, the Gero family ranks as one of the five most powerful clans of Masters, commanding unparalleled skill and influence. Hikaru Gero, heir to this notorious lineage, has spent his life hidden in the underworld's shadows, far removed from love or marriage. However, to prevent the Poison Master bloodline from dying out, the head of the Gero family declares that Hikaru's sister will be forced to bear an heir. Soon after, Hikaru encounters Mei Kinosaki, a brilliant marriage swindler who also happens to be his current assassination target. Determined to protect his sister and resolve the succession issue himself, Hikaru makes an unexpected request on the spot—asking Mei to help him get married. "...That's the first time anyone's ever proposed like that." And so, with the cunning marriage swindler Mei as his advisor, the elite assassin Hikaru embarks on the toughest mission for love. His goal? The ultimate marriage. The most unlikely duo—an assassin and a marriage swindler—take on the world's toughest mission for love!