In Victorian-era London, on the night of Ciel Phantomhive's tenth birthday a fire destroyed his manor and killed both of his parents. In a moment of death Ceil strikes a deal with a demon, his soul in exchange for revenge. This demon becomes his butler and calls himself by name of Sebastian Michaelis, to protect and serve Ciel until the deal has been completed. One month after the fire Ciel returns from being missing, with his new butler Sebastian. Ceil is now the head of the Phantomhive corporation, handling all business affairs as well as the work as the Queen of England’s guard dog and looking for his parents murders.
Humans live in the world of Assiah, demons in Gehenna. The two dimensions are not meant to interfere with each other, but demons still possess creatures in Assiah in spite of this. The humans who can fight these demons are known as exorcists. Rin Okumura is a boy who bears the curse of being Satan's illegitimate son. His foster father sacrificed himself to save him from demons. To avenge his foster father's death as well as to prove himself, Rin decides to follow the path of an exorcist.
Isei finds new troubles about to enter into his life and two even sleeping in his bed. While looking at an old photo album of Isei's, Kiba sees a familiar sword being held by an old friend of Isei's, which reopen's old wounds. Meanwhile, Devils, Fallen Angels, and The Church's agents all clash to get their hands on, or just destroy, powerful weapons called Holy Swords. If that weren't enough, Isei's red dragon warns him that he senses his old rival the white dragon is drawing near.
The story follows Issei Hyōdō, a dim-witted, lecherous second-year high school student who is killed by a girl on his first date ever. Issei is reincarnated as a devil, and from that day forward, he serves as an underling of Riasu, a high-level devil who is also the prettiest girl on Issei's campus.