Fionna and Prince Gumball are in the Graveyard doing a science experiment aimed to bring dead Candy People back to life while Cake and Lord Monochromicorn are playing in the are playing in the forest. He tests his serum on a dead Ms. Creampuff. Hovever she is resurrected as a candy zombie that craves sugar. Ms. Cream Puff then spills the serum all over the graves of the dead Candy People, and they rise as zombies too.
Prince Gumball tells Fionna that the Candy Zombies will go to the Candy Kingdom and consume the Candy People. At the kingdom, Prince Gumball summons all of the Candy People to the main candy foyer, where he announces that they are all going to have a slumber party. He explains to Fionna hat if the Candy People find out about the zombies, they will explode in fright (although they do not explode in later episodes when they become afraid). At the same time, Starchie searches for Prince Gumball in the Graveyard to give him a bigger corpse shovel, but she explodes upon seeing a Candy Zombie.Back at the Candy Kingdom Gumball makes Fionna royal promise not to tell anyone about the candy zombies, then gives her orders to keep the people distracted while he tries to finish the serum in he tries to finish the serum in his lab. Cake hears part of the discussion and begins asking what Prince Gumball and Fionna were up to. In an attempt to keep the promise, Fionna avoids avoids answering Cake and tries playing Truth Or Dare, where Chocoberry dares Ms. Cupcake to take off her wrapper. Ms. Cupcake then asks Cake whether she prefers chocolate or fudge. Cake says she cannot eat either because she is a cat, and if she eats either she would probably die. But says if she had to choose, she would choose neither fudge nor chocolate. Finally, Cake dares Fionna to tell the truth about what's going on. Fionna tries to dodge Cake question by starting a game of Dodge Socks, but when she throws her sock, Cake smells Ms. Cream Puff's scent on it, because Fionna kicked the zombie Ms.Cream Puff when she attacked Prince Gumball earlier. To make sure Cake does not find out about the zombies, she gets Cake to play Seven Minutes in Heaven with Lord Monochromicorn. Meanwhile, the zombies have arrived, and Fionna tries to keep the Candy People ignorant by drowning out the zombies' noises with loud music and barricading all the exits and windows. Fionna gets the Candy People to help her keep the zombies out of the castle by making up a game called Blockado, "the game of barricades." Knowing the blockades will not last long, Fionna gets another idea when she he takes a look at Mandy the talking piñata. She tells the Candy People that they're gonna smash piñatas (not Mandy)and blindfolds them so they do not see that the piñatas are really zombies breaking into the castle. Soon, they beat up all the zombies and start eating the candy that was stashed in them. Cake and Lord Monochormicorn come out of the closet and see all the dead Candy Zombies. When Fionna tells her hat happened, time suddenly stops. The only people who are not frozen are Fionna and Prince Gumball, who hasn't finished his equation yet.Suddenly, two Guardians of the Royal Promise arrive and grab Fionna and Gumball. For breaking her royal promise, the guardians plan to punish Fionna with a trial by fire. However, since Gumball cares for Fionna they decide to make her solve math questions instead. Fionna is terrible at math, and at first she is given a complicated math problem but luckily one of the Guardians changes it to 2+2. Fionna answers correctly and the Guardians reset time. Gumball realizes that 4 is the last figure he needs to perfect his decorpsinator serum, as he was to smart to see such a simple answer. Fionna annd Gumball bring the zombies back to life, including Starchie, and the Candy People reunite with their former dead relatives. Cake tells Fionna that she could have her that she had made a royal promise to not tell about the zombies, since Cake knows "what's up. Fionna promises Gumball to keep her promises from now on, and Starchie (despite not being a zombie anymore), craves for flesh and bites Fionna's leg. Fionna squeezes her and Starchie burps.