In late 1999, a dying MugenDramon (Machinedramon) and Chimeramon (Kimeramon) jogressed at the last possible instant. The jogress was never supposed to occur. The result, Millenniumon, caused a time slip reviving the villains of Adventure, and he captured the Chosen Children and their Digimon. Only Agumon escaped. Ryo was called to the Digital World for the first time, and laid the smack down. Which, shouldn't have happened, Millenniumon reflected, unless Ryo had the same powers as he. Later, Millenniumon revived. Ken and Ryo went to the Digital World, and Ken became a Chosen Child and found Wormmon. Millenniumon was defeated yet again. Even later, the Holy Beasts set up a tournament of Chosen Children to "determine the strongest". Of course, the entire thing was rigged, and Ryo learned that everybody he considered a friend has betrayed him to train him up to take on Moon=Millenniumon. Ryo managed to defeat Moon=Millenniumon, who informed us that Ryo was the Light and Millie the Shadow. Moon created an explosion that ripped through spacetime and sent Ryo to the past.
Meanwhile, Moon was released from his crystal prison, and was now Zeed Millenniumon. He broke the seal in the Adventure world so Apocalymon could cross the Wall of Fire, and arrived in the future, where he promptly conquered all the Digital Worlds, experiemented with cloning, and created numerous Digimon, such as Deathmon, Imperial Dramon Paladin Mode, Dukemon Crimson Mode, and Apocalymon. He then set his sights on the past, so he could have complete control over everything ever.
However, ENIAC, in the distant past, would not allow this. He sent Ryo off to destroy Millie's minions, but in the end, ENIAC himself was shut down. All that stopped Zeed now was Atanasoff. In a last desperate gamble, ENIAC sent Ryo to Zeeds castle in the future. In the end, Monodramon and Zeed jogressed. The end result was Cyber Dramon.
Ryo, for whatever reason, did not want to or could not return to the Adventure World. He went to the Tamers dimension. He either brought his father there, was adopted, or had a counterpart there (think Adventure and V-Tamer taichi) who ran away, leaving his father there, who accepted the Adventure Ryo as his own.
XROS WARS (Young Hunters)