It’s Fionna’s birthday! Prince Gumball and Marshall Lee throw her a surprise party that gets out of hand when an experimental serum turns the birthday cake into a monster. 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! Join writer Kate Sheridan (Fallow Time) and artist Vivian Ng (Nickelodeon The Legend of Korra - Turf Wars) on a brand-new adventure in the incredible world of Cartoon Network’s Adventure Time.