After losing his parents to a demon attack when he was young, Kiyoshi Harai dedicated his life to becoming an exorcist so that no one else would suffer the same fate. He graduated as the valedictorian of his class at the Exorcist Academy, the youngest student to ever do so, and is widely considered the most powerful exorcist in history. Despite his overwhelming strength, Kiyoshi has a critical weakness an extreme shyness and deep-seated fear of demons that contradicts his immense power.

The story is set in Naraku City, where Kiyoshi is assigned to work at a local branch of the Exorcist Association. The branch operates out of a Chinese restaurant called the Gates of Hell, run by his mentor and savior, Tenma Sakaki. While working as a chef making omurice, Kiyoshi and his peers fight to exorcise demons and protect the weak, balancing high school life with their dangerous duties. Kiyoshi carries a small notebook listing 100 things he wants to achieve in high school, reflecting his desire for normal adolescent experiences despite his extraordinary abilities.

Kiyoshis exorcism techniques are based on solar and lunar power, utilizing Crest Xr-Cism abilities from his Little Miss Sunshine and Moonrise Kingdom crests. Although officially ranked as a Grand Cross exorcist, Kiyoshi is already acknowledged to possess strength on par with a Cross Paladin, the highest rank an exorcist can achieve.

The narrative unfolds through several major arcs. The Introduction Arc sees Kiyoshi arriving in Naraku City and joining Tenma Saki branch as demonic activity increases, including encounters with Demon Lords like Meto and Sata. In the Jack Joe Arc, Kiyoshi befriends a classmate named Nehan Sanzugawa, who has ties to the Demon Lord Jack Joe, leading to complications when zealous exorcists appear to eliminate the demon.

The Grand Cross Assessment Test Arc follows Kiyoshi and fellow exorcist Sosuke Hitsugi as they participate in a rigorous examination to achieve the Grand Cross rank, encountering other participants such as Zangi Tosaka. During the Demon Lord Surface Expedition Arc, a school club president creates a legitimate summoning sigil for Asuta Roth, one of the Three Great Demon Lords, leading to a major confrontation.

Following this battle, the Goma Hot Spring Arc depicts the Naraku Branch recovering and training at hot springs, where they encounter the Knights of Light, a group of elite exorcists who join their training. The Black Parade Arc, the longest arc of the series at thirty chapters, begins when exorcist Kyoka Kurogane takes an interest in Kiyoshi, and the Black Parade event commences a week later, representing a major turning point in the series.

The current storyline enters the Demon Realm Infiltration Arc, the first arc of the Helze Family Saga. After Kiyoshi loses his Precept Power following his fight with Asuta Roth, he undergoes recovery at a specialized hospital. Learning that Commander Goro Shishido has gone missing in the Demon Realm, Kiyoshi and Sosuke join the Demon Realm Special Forces on a mission to find him. This arc expands the series mythology significantly, introducing the Great Demon Lord Helze Bub, exploring the consequences of Roths death on demon realm politics, and revealing that Kiyoshis compassion extends even to demons as he befriends a kind-hearted demon named Yamada who teaches him that not all demons are evil.