The following information may contain Spoilers. Please proceed at your own risk.
Jesse, Petra, Lukas and Ivor enter a new dimension inhabited by Zombies, with two moons hovering in the sky instead of one. They find an invitation signed by an individual called "The Host", inviting them to his/her mansion to take refuge from the zombies. They run to the mansion, which they enter through a window on the second level.
In the kitchen, the Gang is met by Captain Sparklez and TorqueDawg. After a short conversation, the lights go out and TorqueDawg is killed by an arrow trap which fired Poison Tipped arrows. Jesse examines the crime scene only to find that the loot is sucked in by a trapdoor.
CaptainSparkles takes the Gang to meet the others in the mansion, including Cassie Rose, Cassis's Cat, Winslow, LDShadowLady, Stampy Cat, DanTDM and Stacy Plays. They are surprised to see their portraits on the wall, and find that TorqueDawg's has been crossed off with a big red X.
The group proceeds to the dining room, where Jesse questions the others about TorqueDawg and the Mansion. CaptainSparkles reveals the murderer is probably after the Enchanted Flint and Steel, but as he sits to reveal this to the others, DanTDM accidentally presses a button causing him to get sucked into the floor and suffocated using Sand. The "Host" then reveals himself to be The White Pumpkin and threatens the group that he will resort even to murder to get what he wants.
Jesse finds that there was a button which activated a recording and the "White Pumpkin" was only a dummy behind a painting. He/she interrogates DanTDM, Stampy Cat and LDShadowLady, and has the choice to accuse Stampy, Lizzie, DanTDM or no-one. If he/she accuses Dan or chooses to accuse no-one, DanTDM will retreat to the Library. If he/she accuses Stampy, the latter will retreat to the library.
Jesse discovers a passageway through the kitchen which contains a lever to fire the Tipped Arrows, and follows it to the Library where he notices the White Pumpkin spying on Dan/Stampy. Determinately, LDShadowLady or DanTDM will visit the characters respectively, and when spiders attack the group, Lizzie or Dan are killed by the White Pumpkin in their respective scenarios. Lukas joins Jesse as he/she chases the White Pumpkin, but the lights go off again, and Lukas is left with the White Pumpkin mask in his hand, leading everyone to believe he is the White Pumpkin and causing them to lock him in a closet. Jesse and Petra then head to explore another secret passageway which ultimately leads them to the White Pumpkin's secret lair, where they find zombie spawners responsible for the zombies inhabiting the dimension, several cats resembling Winslow and a chest full of crossed-out paintings. Jesse notices that the only one missing from the paintings is Cassie.
Back on the top, Jesse can accuse Cassie of being the White Pumpkin, and if he/she provides enough evidence, Cassie will admit it and will ultimately end up drowning in Sand, similar to CaptainSparklez. At this time, the Sun comes out, and the Zombies burn away and the Youtubers and Jesse's Gang part ways. As Jesse goes to light the portal back to the Portal Room, a trap is activated leading to an underground section and it is revealed Cassie is alive. The two fight over the Flint and Steel, however, Jesse is able to overpower her with the help of the Gang and Cassie falls into her own trap. She finally beckons for Jesse to give her Winslow, and Jesse can decide whether to do so or not. With the mystery solved, Jesse and the Gang head back to the Portal Hallway.