Once upon a time, a girl skipped a stone across a lake, and hit a unicorn in the face. In gratitude (yes, that makes sense), the unicorn grants her a wish. A "something I can do for you" sort of wish, not a wishing ring sort.
Phoebe wishes for the unicorn to be her best friend. And Marigold Heavenly Nostrils has to admit that's in her power.
Once Phoebe gets past her resentment that having a unicorn best friend doesn't make her awesome (as Marigold points out, she didn't wish to be awesome), they have adventures about spelling bees, a record store, a mystery, a birthday party for another girl, meeting Phoebe's parents, and more.
