I Wonder Which Wonder Witch Is Which

Harry, Ron, Hermione and Ginny are the testers of a new Wonder Witch potion, which allows the drinker to see their lover as someone else during intimacy, so the foursome can satisfy their extramarital affairs without actually cheating on each other. However, the creator of the potion is the infamous Romilda Vane. Hilarity, drama and utterly shameless smut ensue.