From the introduction: Welcome to Earth! Well, possibly Earth. Except this Earth isn’t alone – there’s a good number of nearby dimensions that are a simple cut with some magic scissors away. One of those worlds is Mewni, where one Star Butterfly of the Butterfly Kingdom just reached the age where she gains the royal magic wand… and gets sent to Echo Creek, where the Diaz family will put her up (and survive the collateral damage) as she learns how to use her newly-gained magical abilities.