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A pair of twins find themselves in a land called Butterfly Valley, where they must help the Butterfly Fairies defend themselves from King Boogieman, who wishes to kidnap and marry Princess Butterfly.
There was a brief controversy when Princess Butterfly married King Boogieman to avoid marrying an evil warlock named Frank, who planned on using her power to take over the world. Many parents saw King Boogieman's sucess in marrying the Princess as teaching children that evil can triumph, a very undesirable children's lesson. The show continued after King Boogieman married Princess Butterfly.
The show's creators made Frank into the show's new villain. Alongside this, the potrayal of King Boogieman became that of a reformed villain, who had changed into a kind, but monstrous looking king as a result of his change in heart. There were other surprisingly dark themes that occurred in the show's episodes, with Season 4 being one of the darkest and most frightening of the seasons.