It's a very special day in the Land of Ooo: Fionna the Human's birthday! To surprise her, Prince Gumball and Marshall Lee have Cake distract Fionna with an adventure as they prepare the biggest surprise party in history. But when Prince Gumball's experimental serum turns Fionna's birthday cake into a monster, everyone must work together to save the Candy Kingdom...and maybe a friendship, too! -- publisher website.