Okay, well, this IS a spoiler, so don’t read if you don’t want to be spoiled.

Basically, it started off with Sora challenging Yuto, demanding to know where Shun was so they could have a rematch.  Yuya followed from a distance and ended up joining Sora against Yuto in an attempt to protect Sora.  Sora is dragged back to the Fusion Dimension in the middle of the duel, and then Yugo shows up.  Yuto and Yugo had already met before and both thought the other was allied with Fusion, and so they started dueling.

Both of them get out their ace dragons, Dark Rebellion XYZ Dragon and Clear Wing Synchro Dragon, who start to resonate with each other strangely.  Both boys’ eyes start glowing and they are suddenly consumed with a need to destroy.  Yuya, who remembered hearing Yuto say he didn’t want to hurt people anymore, tried to snap Yuto out of it.  He managed to get through to Yuto, who snapped out of his “dragon rage”, but Yuya was not quick enough or close enough to Yugo to snap him out of it too, and Yugo attacks with Clear Wing, defeating Yuto.

After that, there’s a weird scene about smiling (EGAOOOOO) (which we will see in a million and one flashbacks like wow I think we remember that happened writers) in which Yuto asks Yuya to try and bring back everyone’s smiles, gives Yuya Dark Rebellion Dragon, and disappears.  Yugo then snaps out of his dragon rage and is like ‘the fuk just happened’ and then I believe he’s teleported away when Yuzu appears to find Yuya.