"Kekkai" is a supernatural action anime set in a world where the boundary between humans and demons has been shattered. The story unfolds in Hellsalem's Lot, a city that was once New York but transformed into a chaotic melting pot of humans, monsters, and otherworldly beings after a mysterious event opened a portal to the demon world. The city is now a lawless zone, governed by a fragile truce between humans and supernatural entities.

The protagonist, Leonardo Watch, is a young photographer who gains the "All-Seeing Eyes of the Gods," a powerful ability that allows him to perceive and manipulate the unseen. This power comes at a cost, as it results in the loss of his sister's eyesight. Driven by guilt and a desire to restore her vision, Leonardo moves to Hellsalem's Lot to seek answers. There, he joins Libra, a secret organization dedicated to maintaining balance and order in the city. Libra is composed of eccentric and powerful individuals, each with unique abilities, who work to combat supernatural threats and prevent the city from descending into chaos.

Leonardo's journey intertwines with the lives of his teammates, including the hot-headed swordsman Zapp Renfro, the stoic and enigmatic chain-smoking Klaus Von Reinherz, and the cheerful but deadly assassin Steven A. Starphase. Together, they face a variety of challenges, from rogue demons and criminal syndicates to ancient conspiracies that threaten the stability of both worlds. The narrative explores themes of sacrifice, loyalty, and the blurred lines between good and evil, as Leonardo grapples with the moral complexities of his powers and the consequences of his actions.

The plot is driven by episodic missions that gradually reveal the larger mysteries surrounding Hellsalem's Lot, the origins of Leonardo's powers, and the true nature of the demon world. As the story progresses, alliances are tested, and the stakes escalate, culminating in a climactic confrontation that forces Leonardo and his allies to confront their deepest fears and desires. The series balances intense action sequences with moments of humor and introspection, creating a dynamic and engaging narrative that keeps viewers invested in the characters' fates.