A few weeks after the end of Tag Tamers, Ryo is talking to a sick Ken, who informs Ryo of an odd poll on the Internet that has questions with answers only he, Ryo, and Taichi would know. Ryo takes the poll, and is pulled into the Digital World by one of the four Holy Beasts to be in the D-1 Tournament.
(Which Holy Beast depends on what one you choose in the last question of the poll; it can be Qinglongmon, Zhuqiaomon, Xuanwumon, or Baihumon. It doesn't really matter in the end, all it changes are the identities of a few bosses and which Holy Beast speaks to you.)
Gennai and Piccolomon inform Ryo that the D-1 Tournament was set up by the Holy Beasts, and whoever wins will partner with the Holy Beast chosen in the poll. Ryo goes to the D-1 Arena, and the other Chosen Children [Not the ones seen in Adventure or Zero Two, FYI, just random people.] mock him, doubting he could truely be the one who defeated Millenniumon, or that if he did Millenniumon was a pushover. Piccolomon arrives to break everyone up, however. Ryo returns to Gennai's home, where he is given a D-Terminal that allows him to communicate with Ken.
Before the D-1 Tournament starts, a Holy Beast [the one chosen in the poll] explains to Ryo the "true" purpose of the tournament. According to him, the other three Holy Beasts had each turned to evil, plotting complete and absolute power for themselves. Realising this one Holy Beast was not in on the plan, they united and turned against him. The lone Holy Beast realised he would need a powerful Chosen Child to aid him, so he started the tournment in the hopes he and a Chosen would be able to defeat the other Holy Beasts and make them see the error of their ways.
And so, Ryo proceeded to the tournament, where he ripped through the competition, which included Taichi, Yamato, and Mimi. Well, Ryo and V-Mon, Wormon, and oddly enough, Agumon ripped through the tournament. (Maybe I forgot and you don't fight Taichi...)
Naturally, the tournament was rigged - Ryo was meant to win all along, and the Chosen Children were just helping him prepare for a battle not even the Holy Beasts could hope to win, as Taichi explains. The Holy Beasts repeat this, and say only Ryo seems to have the power to defeat this wicked god. Of course, the god referred to is none other than Millenniumon, whose body was destroyed and spirit imprisoned, but just the same he is reviving as Moon=Millenniumon. The Holy Beasts leave him near Moon=Millenniumon's area, and a mute Ryo battles through Millenniumon's minions. At last, he comes to Moon=Millenniumon.
Moon tells him that he and Ryo share the same power, and this is why only Ryo can defeat Millenniumon. They battle, and Ryo manages to come out on top, although before he can finish Moon off, he is told that they are shadow and light, neither can exist without the other, and if Moon dies he is taking Ryo with him. Moon tells him to fly with him in the world of time, and creates a massive explosion that rips the space-time continuum apart, sending Ryo to the distant past and Moon to the distant future.
Later, when no-one has heard from Ryo, Ken reminds Gennai that Ryo has always managed to return from Millenniumon's clutches before. Weeks pass, and still no Ryo. But Ken believes Ryo is on a new adventure, and that unlike him, Ryo is a true Tamer. And at last he says, "No matter what happens on what world, so long as there is a Digital World, surely there's someone who needs Ryo-san's help. Thus, I think it's true. I hope, no matter how many years it takes, that we will meet again.", and the last chapter of Ryo's life in the Adventure world ends.
(Please note that that line was translated by Ajora, not myself. Her site is To Bring Back Yesterday. Go there, or die.)