The mysterious Cadis Etrama di Raizel, also known as "Rai," has been enrolled in Ye Ran High School by his servant Frankenstein to stay hidden from the mysterious organization, the Union, who are out for Rai's blood. Rai is quickly thrown into his new life as a student, making friends and getting used to the daily activities of humans. However, his peaceful life is soon disrupted when the Union abducts his new friends, and Rai is forced to save them.
M-21, a Union agent who went rogue during Rai's rescue operation, joins the Ye Ran High School security staff after being offered a position by the school's director, Frankenstein. On the surface, M-21 appears to be a proper employee, but in reality he is still connected to the Union and the consequences of betraying them.
Nobles Regis K. Landegre and Seira J. Loyard also enroll in the same school to investigate the Noblesse. As the Union searches for M-21 to get information about their missing agents, Rai must keep his identity hidden while protecting his friends and family.