"Black Gate" is a dark fantasy anime series set in a world where humanity is on the brink of extinction due to the emergence of monstrous creatures known as "Demons." The story centers on a young man named Allen, who lives in a fortified city protected by a massive barrier called the Black Gate. This barrier is the last line of defense against the Demons, but it is slowly weakening, threatening the survival of the remaining human population.

Allen, an orphan with no memory of his past, discovers that he possesses a unique ability to wield a mysterious power known as "Eclipse," which allows him to combat the Demons. This power is tied to a legendary artifact called the Black Key, a weapon said to be capable of sealing the Black Gate permanently. As Allen begins to uncover the truth about his origins, he learns that his existence is intertwined with the fate of the world and the Black Gate itself.

The narrative follows Allen as he joins a group of skilled warriors known as the "Gatekeepers," who are tasked with defending the city and researching ways to strengthen the Black Gate. Among the Gatekeepers are key characters such as Lina, a determined and resourceful fighter who becomes Allen's closest ally, and Victor, a seasoned veteran with a deep understanding of the Demons' origins. Together, they embark on missions beyond the safety of the city, facing increasingly powerful Demons and uncovering secrets about the Black Gate's creation.

As the story progresses, Allen encounters a faction of humans who believe that the Black Gate should be destroyed to allow humanity to evolve and adapt to the new world dominated by Demons. This ideological conflict adds complexity to the narrative, as Allen must grapple with the moral implications of his role as the wielder of the Black Key. The series explores themes of survival, sacrifice, and the blurred line between humanity and monstrosity, as Allen and his companions confront the harsh realities of their world.

The plot builds toward a climactic confrontation as the Black Gate's deterioration accelerates, and Allen is forced to make a pivotal decision that will determine the future of humanity. The series concludes with a resolution that ties together the mysteries of the Black Gate, Allen's past, and the true nature of the Demons, leaving a lasting impact on the world and its inhabitants.