Rin Okumura, a 15 year-old boy living in a monastery, begins to worry about his future when his twin brother, Yukio, leaves to start his academic career at the prestigious True Cross Academy. Sent to an interview for a part-time job by his foster father, Shiro, Rin ends up wreaking havoc in the store when he encounters a demon-like creature.

Rin is thrown into turmoil when Shiro tells him that he's the son of Satan! Warning him that he must never draw it, Shiro gives Rin the Koma Sword, and hides him in the monastery basement. Outside, a fierce battle begins...

Vowing to become an exorcist, Rin is taken to True Cross Academy by the mysterious Mephisto Pheles, a friend of their late father. The colossal, fortress-like school; Yukio at the top of the class - it's all so mind-boggling for Rin! Later, Rin heads to the Exorcist Cram School, as Mephisto strictly warns: "You must keep that you're the son of Satan confidential."

Yukio sets out to the school's Exorcist-only shop to stock on some supplies along with Rin. There they find that the owner's daughter, Shiemi Moriyama, is being afflicted by a demon that dwells in her family's garden and, to save her life, they confront it by themselves.

Having Shiemi as a new classmate, Rin's total disregard for classes brings forth the anger of another classmate of his, Ryuji Suguro, aka "Bon". After learning of Bon's story, Rin finds that they have more in common than he would like to admit.

Rin and Yukio find that Ukobach, a cooking familiar appointed by Mephisto Pheles to aid them, is responsible for the meals that are prepared for both of them each day. They also learn how Ukobach can be dangerous when someone disrespects his cooking.