Doesn't matter, en passant is forced

I don't get it, can't black just move the king?

Edit:

OK, so I looked it up, en passant means that if a pawn has just moved 2 spaces and landed next to an enemy pawn, the enemy pawn can move to take the first pawn by moving behind it instead of into the current occupied square. That's news to me and I can't believe this isn't used more often.

That and I mixed up the king and queen because my set had the butt crown on the queen.