The story revolves around Rin Okumura, who, along with his younger twin Yukio Okumura, was raised by Father Shiro Fujimoto, an Exorcist. One day, Rin learns that he and Yukio are the sons of Satan. Witnessing Shiro dying to protect him, Rin draws the demon-slaying sword Kurikara (倶利伽羅), which restrains his demonic powers. From that moment on, Rin not only gains demonic features like fangs and a tail but also the power to ignite into blue flames that destroy almost anything they touch.

Rin wishes to become an Exorcist like his guardian to become stronger and to defeat Satan. He enrolls at the prestigious True Cross Academy (正十字学園 Sei Jūji Gakuen), an exorcist cram school, which is actually the Japanese branch of the True Cross Order (正十字騎士團), an international organization dedicated to protecting Assiah (human realm) from the Gehenna (demonic realm). Much to his surprise, Rin finds that Yukio is already a veteran Exorcist and is one of his teachers. Thus begins Rin’s journey to become an Exorcist, accompanied by his brother and his fellow students who quickly become his close friends.

Rin Okumura bears the curse of being Satan’s illegitimate son. He decides to hide his origins and enrolls at the Exorcism Cram School, a training institute for exorcists located on the True Cross Academy grounds. During an attack by Amaimon, the King of Earth, his identity as the Son of Satan is revealed. His friends are terrified and start to distance themselves. Then someone steals the Left Eye of the Impure King that had been sealed away in the deepest part of the academy. Rin and the others find themselves in an unexpected crisis.

The Demon Lord Satan seeks to conquer the world of Ente Isla by annexing its four continents with the help of his demon generals Alciel, Lucifer, Malacoda, and Adramelech. After being confronted by the Hero Emilia Justina and her companions after they killed Malacoda and Adramelech, Satan Jacob and Alciel escape the world of Ente Isla through a gate to modern Tokyo, Japan. However, due to the lack of magic in the modern contemporary world, both Satan Jacob and Alciel change into forms representing what they would look like if they were human. In order to survive, Satan Jacob takes a part-time job in a fast food restaurant named MgRonald while Alciel serves as his houseman. One day, Satan Jacob, who now goes by Sadao Maō, meets a girl who is actually Emilia in the form of Emi Yusa. The story then unfolds and explores the personalities of each of the characters and their moral values. More characters show up from Ente Isla and they too face the new world dilemmas, often comically.