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.
To add to this, only Yuto thinks Yugo’s allied with Fusion (thanks to his name, as the dork is forced to correct others on the pronounciation lol); Yugo just thinks Yuto’s behind whoever took Rin (”someone I love”) from him. Later, we find out it was Yuri who took Rin, and so Yugo’s teleported to Heartland, where he meets Yuto, except he mistakes his face for being Yuri’s while Yuto mistakes him for being with Academia (given the destruction going on, it’s natural that it’d be his first guess), and so they clash. All while Yuri watches nearby.
And then comes that duel where we get EGAO AD INFINITUM.
So, this all started with a misunderstanding.
FUNNY, RIGHT?