Six years ago, Kamui Shirou and siblings Fuuma and Kotori Monou were inseparable childhood friends. After the sudden, gruesome death of the Monou siblings' mother, Kamui was taken away from Tokyo by his own mother. In the years that passed, the two lived peacefully while Kamui strengthened his innate telekinetic abilities. When his mother passes away in 1999, her dying wish prompts him to return to Tokyo to face his "destiny."

When Kamui reappears in the city, his attitude has taken a drastic turn, and he displays the growth of his powers through violent confrontations in the streets. Various opponents come after him asking if he is "Kamui," but they seem to be demanding something far more significant than his name. These fighters represent two opposite factions—the Dragons of Heaven and the Dragons of Earth—who each wish to recruit him to their cause. Piecing together clues about the apocalyptic "promised day," Kamui realizes that he holds the fate of the world in his hands. Depending on which group of Dragons he allies himself with, he can either choose to save the world—or end it.

X is written and illustrated by the four-person team of CLAMP. It is currently incomplete, but CLAMP has stated their intention to conclude the series one day, if possible. It started serialization in Kadokawa Shoten's Monthly Asuka shoujo manga magazine in March 1992 and stopped serialization in March 2003. Events depicted in the manga were too similar for comfort to real-life disasters that occurred in Japan, which contributed to the series' halt.

Chapters of X have been collected in eighteen and a "half" volumes, out of a total of twenty-one volumes expected. X has been adapted into various anime that speculate about events that occurred before the manga started or after the point where the manga left off.

The series was published in English by VIZ Media as 3-in-1 omnibus volumes from November 8, 2011, to August 13, 2013.