It's All Hallow's Eve and the kids are trick-or-treating and everything is normal, but something magical is afoot, and turns people into whatever costume they are wearing. If you're dressed up as a werewolf, for example, you'll become an actual werewolf. You'd be surprised just how often this happens in fiction. Not to be confused with Becoming the Mask or Evil Costume Switch. Can overlap with The Mind Is a Plaything of the Body, which is bad news. Related to Evil Mask. May be the subject of a Halloween Episode. Examples of Becoming the Costume include: