The central focus of Kekkai Sensen is on the heroic exploits of "Libra," a crime-fighting organization that engages in battles against various paranormal entities in Hellsalem's Lot, which was once New York City. This bustling metropolis was transformed into a supernatural hub after a mysterious portal was unleashed, fusing it with monsters, magic, and mundane existence. With the threat of terrifying entities rampantly roaming the city, it's up to Libra to prevent these horrors from seeping out into the rest of the world. But, when an unassuming Leonard Watch arrives in Hellsalem's Lot, he soon finds himself swept up into the dangerous world of Libra where chaos reigns supreme.