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 were involved in the battle between the Dragons of Heaven and the Dragons of Earth one by one. Here is the summary on the plot from the movie:

At the close of the millenium, the 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 Dragons of Heaven, defenders of humanity, stand ready to protect the world from the Dragons of Earth, the seven angels of legend who embrace the eradication of mankind to bring about the planet's 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 grave peril that Kamui decides to step into his fated position in the climatic struggle of the Year of Destiny: 1999.