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. Still upset by the unfair turn of events, Rin is called out by Shiratori, whom he fought with earlier.
Rin is thrown into turmoil when Shiro tells him that he's the son of Satan! But this doesn't stop the demons from unleashing a brutal attack. 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 - it's Shiratori, who's morphed into a giant demon, versus the exorcists! Just after Shiro manages to end the battle, Rin turns on him in a rage: ""Don't act like you're my father!""
Vowing to become an exorcist, Rin is taken to True Cross Academy by the mysterious Mephisto, a friend of Shiro's. 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 warns: "No one can know that you're the son of Satan."
The first semester has ended, signaling the start of summer vacation for True Cross Academy students. But there's no rest in sight for Rin and the other ExWires: first they must take part in a 3-day drill to prepare them for actual combat, under the guise of a camping trip out in the forest.
Suguro's joint strategy pays off, and the ExWires have success in their sights! But then a gigantic moth demon (chuchi) appears, and attacks Rin the moment he lets his guard down!
Consumed by the power of the flames during his battle with Amaimon, Rin goes berserk! He regains control of himself once his power is exhausted...only to be captured then and there by the current Paladin, Angel!
As the hearing drags on, the Order finds itself unable to decide on Rin's fate, as Mephisto has proclaimed him to be "a weapon for counterattacking Satan." Just then, something happens...!
Hordes of demons are spilling out of Gehenna Gate! And these intruders from the "Empty World" are now indiscriminately attacking the people of True Cross Academy Town! Meanwhile, on the roof of the school building, Rin has been rescued by Suguro and Shiemi and is starting to come to. What Rin sees when he awakens is...
Satan has transformed himself into a gigantic Gehenna Gate, and is attempting to absorb all of Assiah by expanding a rip in the dimension. Rin and Yukio, joined by Shura and Angel, attack the gate, but it's an uphill battle against Satan's overwhelming power.
Meanwhile, at the school, the exorcists are in a desperate situation, as a myriad of demons attack. Just then, Suguro proposes a strategy for a comeback! But will it work? Rousing themselves into action one last time, Rin and Yukio head toward the gate. To protect their beloved friends, and this world!
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