At some point in the past, creatures began to appear in this world. Youkai, monsters, demons. The creatures that ate children and reduced men to silent chunks of meat were called 'oni'.
Those that squared off against them in secret were called 'exorcists'. The organization that supported them was called 'the Guild'. Their existence will never be made open to the public, but at times they have been known to possess influence sufficient make the representatives of the time move like dolls at will.
The time is the present. A young man who lives as a normal student, Sakagami Homura. He was kidnapped by someone and when he came to he was riding on a giant ferry. Those who appeared before him were a man with a poor attitude, Kajimae; three girls who named themselves 'exorcists': Mai, Anya, and Kaho; as well as the talking sword, Onitorimaru.
'You are the same as us, so take up the sword that kills oni, Onitorimaru, and kill oni.'
A bewildered Homura gains the aid of the pure-hearted girl, Chizuru and attempts to escape, but he fails. What appeared before him at that time was a real 'oni'. The battle in which he was dragged into, and the sword 'Onitorimaru' which resonated with him. By the time the oni hunt was over, Homura realized that his fate was in the midst of changing.
At the coastal island he arrived at, Homura ends up living in the same dorm, as well as going to the same school as the three exorcist girls. Chizuru, who had become his classmate as well, was good medicine for Homura, whose life had been changed beyond recognition. While awkward at first, Homura and the others begin to understand one another amidst living their daily lives.
Just when they are becoming accustomed to their peaceful daily lives, Chizuru is attacked by an oni. The fleeing students, a classmate who was a friend only moments before is killed and eaten by an oni. In order to survive, to protect people and his friends, and to save his beloved, Homura and the others' fight begins.
[Drawn and translated from official game website]