In a world where humans possess various unique abilities and coexist with different races, a boy named Kail, who was treated as a dropout due to his lack of ability, awakens to the strongest unique ability after becoming a retainer to a vampire!! This power allows him to adapt to any demi-human's characteristics and, despite being a retainer, exhibit strength surpassing his master, becoming the 「Strongest Retainer」!! Suddenly gaining attention, Kail begins to be approached by various races— 「You have the potential to become the king of all races.」