A story in which the characters we know are seen in a reality that's somehow different, often disturbingly so. If they can access multiple alternative universes at the same time, that's The Multiverse. Sometimes everyone has an Evil Twin. Other times, everyone has a twin that's just a little different. Allows the goodies to be baddies for an episode, or for half of the cast to be killed — but not really. Sometimes it's just part of Side-Story Bonus Art.