According to the legend, five hundred years ago, there was an insignificant lord who possessed a mysterious power, this power draws ayakashi (supernatural creatures) to him. A demon exterminator, Tokimori Hazama was called upon to protect the lord and the castle. The lord's power stays on the land even when he had died. Thus, Tokimori founded the Hazama clan, who inherits his techniques, to protect the land for centuries to come. This land, is Karasumori.
In the present day, Yoshimori Sumimura and Tokine Yukimura, heirs of the Hazama clan, are the kekkaishi that protect Karasumori. They are ability users (people who can use supernatural powers) who use a technique called Kekkai. Kekkai is a form of magical barrier to capture and destroy ayakashi that are drawn to this shinyuuchi (sacred land). Any ayakashi that stay on the land become stronger. Yoshimori and Tokine are to guard the land from the intrusion of ayakashi who intend to power up there.
Yoshimori and Tokine suffered a lot of hardships in their responsibilities to protect Karasumori. The ayakashi they must fight are becoming more and more powerful, but they managed to protect the land with the help from Yoshimori's older brother, Masamori Sumimura, and Urakai. Urakai itself is an organization of ability users that is governed by a council of twelve, consisting of high level ability users.
Later, it's revealed little by little that the legend is full of lies. The real cause of the ayakashi power up is Chuusinmaru, an illegitimate son of the Hazama clan's founder, Tokimori Hazama, with the Karasumori clan's heiress who was named Tsukikage. Tokimori used forbidden arts to grant his son unearthly power, hoping that his son would grow into a great man who can conquer the world. However, the plan backfired, as Chuusinmaru was granted a power to draw people's life force, killing everybody. Tokimori has no choice but to seal his own son, who has become a too powerful and dangerous existence, in the Karasumori land, but the seal isn't perfect, as Chuusinmaru's power still leaks out, powering up ayakashi who stay at the land.