The world of Blue Exorcist consists of two dimensions, attached to each other as a mirror and its reflection. The first is the material world where humans live, Assiah, and the other is Gehenna, the world of demons, which is ruled by Satan. Originally, a journey between the worlds, or even a contact between them, was impossible, however, any demon is able to pass to the dimension of Assiah through the possession of a living being in it. Even so, demons have historically wandered among humans unnoticed, visible only to people who have had contact with demons before. In contrast, there are Exorcists, people who train to destroy demons who act in a damaging manner in Assiah. With more than two thousand years of existence, this group has several branches all over the world being secretly under the command of the Vatican itself and acting in a clandestine manner.

You, who, along with your younger twin Yukio Okumura, was raised by Father Shiro Fujimoto, an Exorcist. One day, you learned that you and Yukio are the sons of Satan. Witnessing Shiro dying to protect you, you draws the demon-slaying sword Kurikara, which restrains your demonic powers. From that moment on, you not only gains demonic features like fangs and a tail, but also the power to ignite into blue flames that destroy almost anything you touch.

You wish to become an Exorcist like your guardian to become stronger and to defeat Satan. You enrolled at the prestigious True Cross Academy, an exorcist cram school, which is actually the Japanese branch of the True Cross Order, an international organization dedicated to protect Assiah (human realm) from the Gehenna (demonic realm). Much to your surprise, you find that Yukio is already a veteran Exorcist and is one of your teachers. Thus begins your journey to become an Exorcist, accompanied by your brother and your fellow students who quickly become your close friends.