Ten thousand years ago, a great and powerful Digimon was defeated and sealed away by the Holy Knights as the "Digi Core". Years later, a virus infected the digital world, leaving only those that possessed the D-Antibody (X-Antibody) remaining. The only members of the Holy Knights remaining are Omega of Power (Omnimon X), Duke of Courage (MedievalGallantmon/Gallantmon X), and Magna of Miracles (Magnamon).

At the start of the series, the "God of Death" MetalPhantomon draws Hikaru, Kiyoshi, Masuken, and Teru into the digital world. Originally, Hikaru is left alone with his Digimon (which starts out as a Dorimon), and has to battle his friends Teru and Masuken, who are under MetalPhantomon's control.

(Source: Wiki)