X (the TV series) basically explains what happened in X 1999 (the movie) in more detail and greater depth. X, the TV series, introduces all the main characters who are involved in the battle of Dragon of Heaven vs Dragon of Earth one by one. Here is the summary on the plot from the movie:

At the millennial edge, the concluding battle for humanity's future is staged. Kamui Shirou's destiny has been decided as he returns to Tokyo to face his ultimate challenge. The Dragon of Heaven, defenders of the Earth, stand ready to protect the world from the Dragon of Earth, the seven angels of legend, who embrace the devastation of the planet to bring about its purification. Now Kamui must decide which side to fight for although he finds the idea utterly unappealing. It isn't until realizing that his two childhood friends, Fuma and Kotori Monou, are in grievous peril that Kamui decides to step into his fated position in the climactic struggle of the Year of Destiny: 1999.

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Alternative version: X
Other: Clamp in Wonderland
Alternative setting: Tsubasa Chronicle
Other: X2 Double X
Alternative setting: Tsubasa Chronicle Dai 2 Series
Other: Clamp in Wonderland 2 1995-2006
Alternative setting: Tsubasa Tokyo Revelations
Prequel: X Yochou