"X" is a supernatural action anime set in a world where the fate of humanity hinges on a prophesied battle between two factions: the Dragons of Heaven and the Dragons of Earth. The story unfolds in modern-day Tokyo, where Kamui Shiro, a young man with immense psychic powers, returns to the city after a long absence. Kamui is the central figure in the prophecy, destined to decide whether humanity will be saved or destroyed. His childhood friends, Fuma and Kotori Monou, are also drawn into the conflict, with Fuma becoming Kamui's rival and Kotori caught between them.

The Dragons of Heaven, led by Princess Hinoto, aim to protect humanity and prevent the destruction of the world. They believe Kamui is their key to victory. The Dragons of Earth, led by Hinoto's twin sister Kanoe, seek to cleanse the world of humanity, viewing it as corrupt and beyond saving. Both factions consist of individuals with unique supernatural abilities, and their clashes are marked by intense battles and emotional confrontations.

As the story progresses, Kamui struggles with his role in the prophecy and his feelings of guilt and responsibility. His relationships with Fuma and Kotori become increasingly strained, especially after Fuma is manipulated into joining the Dragons of Earth. The narrative explores themes of destiny, sacrifice, and the moral ambiguity of choosing between the greater good and personal desires. The series builds toward a climactic confrontation that will determine the future of the world, with Kamui forced to make a final, irrevocable choice.

The anime features a large ensemble cast, including other key members of the Dragons of Heaven, such as the stoic swordsman Subaru Sumeragi and his twin sister Hokuto, as well as the enigmatic Sorata Arisugawa. The Dragons of Earth include characters like Seishiro Sakurazuka, a ruthless assassin with a mysterious connection to Subaru. The interplay between these characters adds depth to the story, highlighting the personal stakes and emotional conflicts that drive the larger battle. The series blends action, drama, and supernatural elements, creating a tense and engaging narrative centered on the struggle for humanity's survival.