It's a genre of anime and manga - well, and light novels, and games, too. Basically, somebody gets dragged into a fictional universe and has to solve a bunch of problems in it, and then they get to go back home when they're done. It's kinda overdone but there are some good classics, if you really are interested in it, I can rec you some stuff...

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But, uh, if we're in one, it's probably more important that we figure out how to get out of it.