In the Rainbow Realm, a magical land ruled by a circle of unicorn princesses, Princess Bloom is busy planning her birthday party in the Magic Garden. When the day of the party arrives, she realizes that she forgot to send invitations! Bloom immediately gives them to the mail-snails for delivery, but she wants to do something extra special for Princess Prism, her best friend. Ernest, the blundering wizard-lizard, casts a spell to make Prism's invitation fly out the window, on its way to Prism--hopefully . . . When Cressida arrives from the human world for the party, she finds that all the unicorn princesses have received their invitations except Prism. And when Bloom and Cressida go to set up the final touches, they find a flock of quails destroying all the decorations! Bloom is devastated--her hard work is ruined and her sisters are mad at her for seemingly excluding Prism. Will Cressida find a way to save the party? One thing is certain--she'll need everyone's help to do so.