That’s right, it’s the video for the end of MS Paint Adventures: Homestuck: Act III. And it’s even more awesome than the one for the end of Act II, if you can believe it.
I hope to god you’ve been reading this comic, because there’s no way I can manage a recap this time. Far too much has happened. Maybe read my recap for Act II, and then read all of Act III? C’mon, it won’t take that long, and it’s totally worth it.
Or I guess you can click this link, and get your summary there, such as it is, crust-dry wiki style and all. But what fun is that? Come on. Read the comic.