It's pretty complicated, but it's kind of like... the magic that keeps their world together is running out, so it'd collapse anyway. But the bad guys are trying to find a way to save some of the people living there while sacrificing all the other ones. Oh, and there's also other bad guys that just want to let everyone get wiped out. Negi-sensei has a plan to keep the world together without having to sacrifice anyone, though, so I'm helping him with that. But it basically means it's just him and the rest of my class versus a whole army.