Akutsu Masatora is a demon who has infiltrated a high school in the human world. His goal is to find candidates to aid Hell in the fight against their natural enemies, the angels of Heaven. Assigned a seat next to Lily Amane, another student who transferred here not long ago, Akutsu falls prey to her beauty and decides to recruit her to his cause. But there's more to Lily than meets the eye, and Akutsu not only finds himself an accomplice to one of the very enemies he is supposed to defeat, but he also might have developed feelings for her.

The "Noblesse" Cadis Etrama di Raizel, also known as "Rai," is enrolled in Ye Ran High School by his servant Frankenstein to stay hidden from the sights of the Union, a mysterious organization out for Rai's blood. Rai commences his life as a student, making himself familiar with his classmates and the daily activities of humans. However, his new life is far from peaceful, and Rai is soon forced to save his new friends from the hands of the Union that had abducted them. Meanwhile, M-21—a Union agent gone rogue during Rai's rescue operation—joins the Ye Ran High School security staff after a proposition by the school's director, who happens to be none other than Frankenstein himself. On the surface, M-21 is a prim and proper employee, but in truth he is shackled by his former ties to the Union and the inevitable consequences of betraying the organization. To further complicate matters, Nobles Regis K. Landegre and Seira J. Loyard enroll in the same school to investigate the Noblesse. While the Union conducts a manhunt for M-21 to extract clues regarding their missing agents, Rai is forced to keep his identity hidden while protecting all that he holds dear. [Written by MAL Rewrite]

At True Cross Academy, Rin Okumura unknowingly comes across the first signs that something is amiss; ordinary people have been gaining the ability to perceive the invisible creatures that roam the human realm, Assiah. While Rin sees this as an opportunity to make a new friend, his twin brother, Yukio, is more burdened than ever. Swamped with his education, exorcist duties, and teaching position, Yukio has remained silent about his awakened powers, unable to understand their scale or significance. While Rin and his friends prepare for the annual Exorcist Certification Exam, the True Cross Order has uncovered the first artificial gate to the demonic realm of Gehenna. Witnessing its size and the advanced technology that made it possible, the exorcists realize they are now dealing with the Illuminati, an extremely wealthy and powerful organization. Moreover, the inscrutable Illuminati have planted three spies in the True Cross Order—and one may be hiding among the aspiring exorcists sworn to protect mankind from the demonic threat.

High schooler Ganta Nakami has trouble falling asleep most nights. As a result, he is irritable at school, always searching for an opportunity to find a secluded place to doze off. On the other hand, Isaki Magari is a free spirit who is well liked by her friends, but no one is aware of her sleep disorder. She makes use of the school's abandoned astronomy club observatory as her secret sleeping bunker when she needs to get some shut-eye. As fate would have it, Nakami finds Magari napping in the observatory. When Magari discovers that she and Nakami have something in common, she offers to share her secret sleeping spot with her fellow insomniac. As the two find warmth in each other's company, the struggles they face start to become easier to confront.