Supersonic monkeys, vampires, talking fishmen, and all sorts of different supernatural monsters living alongside humans—this has been part of daily life in Hellsalem's Lot, formerly known as New York City, for some time now. When a gateway between Earth and the Beyond opened three years ago, New Yorkers and creatures from the other dimension alike were trapped in an impenetrable bubble and were forced to live together. Libra is a secret organization composed of eccentrics and superhumans, tasked with keeping order in the city and making sure that chaos doesn't spread to the rest of the world.
Pursuing photography as a hobby, Leonardo Watch is living a normal life with his parents and sister. But when he obtains the "All-seeing Eyes of the Gods" at the expense of his sister's eyesight, he goes to Hellsalem's Lot in order to help her by finding answers about the mysterious powers he received. He soon runs into Libra, and when Leo unexpectedly joins their ranks, he gets more than what he bargained for. Kekkai Sensen follows Leo's misadventures in the strangest place on Earth with his equally strange comrades—as the ordinary boy unwittingly sees his life take a turn for the extraordinary.
There exist creatures of darkness and evil that plague the night, devouring any human unfortunate enough to be caught in their grasp. On the other side is Hellsing, an organization dedicated to destroying these supernatural forces that threaten the very existence of humanity. At its head is Integra Fairbrook Wingates Hellsing, who commands a powerful military and spends her life fighting the undead.
Integra's vast army, however, pales in comparison with her ultimate weapon: the vampire Alucard, who works against his own kind as an exterminator for Hellsing. With his new vampire servant, Seras Victoria, at his side, Alucard must battle not only monsters, but all those who stand to oppose Hellsing, be they in the guise of good or evil.
In a battle for mankind's survival, Hellsing Ultimate proves that appearances are not all they may seem, and sometimes the greatest weapon can come in the form of one's worst nightmare.