Dipper and Mabel get shipped to their grunkle Stanford Pines who owns a tourist trap called the Mystery Shack. After finding Book 4, Dipper finds signs that may lead to certain death.
The episode starts with the twin's parents shipping them away on a bus. The bus stops at a tourist trap called the Mystery Shack, and their grunkle appears behind a ball of smoke, points his staff at the Mystery Shack, and the S falls off. Mabel then says that she thinks he has wizard powers, which Dipper says is nonsense as there is no such thing as monsters. Stan then leads the kids into the Mystery Shack, where a boy and a girl greet themselves to be named Wendy and Soos. Everyone shakes hands, and Stan has the kids carry their bags upstairs, as they have a lot of work to do. After placing everything in their room, the twins go downstairs. Stan tells Dipper to put up signs in the forest, and everyone else starts watching tv. Dipper sighs, and walks into the woods. He puts up the signs, and as he walks, he falls into a hole which appeared to be just regular ground. Dipper sees signs saying TRUST NO ONE, and GRAVITY FALLS IS NOT WHAT YOU THINK. Dipper walks down a dark hallway lit up with torches, and reaches a room with a desk lit up with a tiny lamp, serving as the only light. Dipper walks over to the desk, which has a book labeled 4 on it. Dipper looks down beneath the desk and sees more of the books, protected with transparent safes. Dipper takes 4, and opens it up. It said stuff about someone named "Bill Cipher", who appeared to be a dream demon. Since he didn't believe in it, he closed the book and starting walking towards where he fell. He found a lever, so he pulled it and appeared where he was when he fell, and it seemed that no time had passed. After putting up the last of the signs, Dipper walked back to the Mystery Shack, and went inside. Dipper saw no one inside, so he looked around more. He started finding signs saying DIPPER, and signs saying KILL BY NINE O CLOCK. Dipper opened another door, with a giant sign saying TRUST NO ONE. Dipper turned around to see his grunkle with a knife. He quickly dropped it, and asked Dipper what he was doing. Dipper then tried getting out, so Stan let him. Dipper then ended up finding lots of blood on the floor, leading to Mabel. She turned around, with a lot of scars covering her face. She said Stan, and pointed at Soos, who was looking at the sign KILL BY NINE O CLOCK. Dipper looked at the clock and watched it strike nine, but nothing happened. Dipper shrugged, and he and Mabel walked upstairs to their room. A mysterious triangle is then seen looking inside, but then quickly disappears. Soos is then heard screaming, and the twins go downstairs to see what happened. Soos says that someone looking at the merchandise got killed, and started screaming again after another person fell lifeless. Soos turned around to see Stan running towards him with a bloody knife, and Soos screams and the episode ends.
- During the end credits, there was a cryptogram that read "WUXVW QR RQH" Once decoded, it reads "TRUST NO ONE"