Oh no! An ancient mummy is haunting the Fermentation Lab, munching on umami-filled ingredients like stinky cheese. This gives John Henson the idea for "umummy" desserts, and he tasks the bakers with creating sweet mummy treats that feature umami ingredients like red miso, aged parmesan, gochujang or soy sauce. Then, it's "bone appetit" for judges Carla Hall, Zac Young and Stephanie Boswell as the bakers get inspired by the lab skeleton, Skelly, to create Skelly's forgotten desserts, where they must use Halloween mini chocolate bars to create skeletal desserts.

The final bakers take on two very difficult challenges inspired by the past -- all leading up to a time-traveling monster mash! In the Dissection Lab, John Henson works on some gross anatomy, and he asks the bakers to create desserts with gummy hearts that will be "dissected" by judges Carla Hall, Zac Young and Stephanie Boswell. Then in the Quantum Physics Lab, John hops into a time machine and journeys to the ancient past for inspirations like medieval vampires and early aliens for time travel cakes. The best baker heads into the future with the Halloween Baking Championship title and the $25,000 prize!

John Henson has a problem in the haunted Paranormal Research Lab, and he's hoping to summon the spirit of Beetlejuice to help broker a deal with the other spirits. To butter up the ghost with the most, John asks the bakers to make black and white desserts using black and white ingredients. Then, John challenges the bakers to create Shrinker desserts that feature a tiny desert on top and large dessert on the bottom. In the end, judges Carla Hall, Zac Young and Stephanie Boswell banish one baker to the cyro-chamber.

John Henson catches a spirit doing some imaging in the X-ray room! The films the spirits have been taking of themselves are monstrous, and John wants the bakers to use these X-rays as templates for build-a-spirit desserts. Then, the bakers enter the lab basement where the portal to the underworld is kept locked behind a secure heavy door, but lately ghouls have been letting all sorts of otherworldly creatures out! The bakers make a portal entremet that reveals an otherworldly creature to judges Carla Hall, Zac Young and Stephanie Boswell.

Ten bakers explore the Chemistry Laboratory at Henson Labs, where the safety rating is negative 35. Lately, the spirits who haunt the lab have been demolishing all the beakers and petri dishes, so host John Henson leans into this devastation and tasks the bakers with creating broken glass desserts that feature sugar glass. Then, there's been a horrible spill in the Chem Lab, so John has a little fun with it and asks the bakers to create sinister bubbling desserts inspired by the toxic mess. Judges Carla Hall, Zac Young and Stephanie Boswell test their poison tolerance before sending one baker to the cryo-freeze.

Things keep turning into other things in the Research and Development Lab, and it's disturbingly delicious. John Henson pairs up the bakers into research teams, and they use exotic candy corn flavors as inspiration for lab specimen-stuffed desserts decorated with tongues, eyeballs, noses or toes. With elimination looming, the bakers serve judges Carla Hall, Zac Young and Stephanie Boswell a mad menagerie of cutesy-to-creepy desserts by turning an adorable stuffy into a delicious, mutated monster.

To celebrate 85 years of the wonderful Wizard of Oz, John Henson is in the lab's atmospheric chamber measuring the velocity of tornado desserts so that the bakers can honor Dorothy's rough trip to Oz. Then, the bakers visit the acid pit, where the spirits have been melting things. Inspired by those who have melted before, especially the Wicked Witch of the West, the bakers make melted witch desserts to appease the spirits, as well as judges Carla Hall, Zac Young and Stephanie Boswell.