Mabel sat happily in a chair near the Mystery Shack's museum area, munching joyously on a sandwich. Just as she got ready to ask a nearby floor-sweeping Soos about how his day had been, she saw something unusual (more unusual than normal) near the corner of the room. The stand that always had a Dodo model on it now had something else entirely. It was a small pigeon model and its label read Arenicolumba pratte.

"Hey, Soos," she said to Soos, now getting his attention, "Do you notice something off here? You think we have thieves?"

"Something's definitely off, Mabel" replied Soos, looking carefully at the extinct pigeon model, "I'm not sure about robbers, 'cause I don't see any fingerprints, but something weirder has got to be happening if models of birds are being replaced."

At that moment, Dipper was strolling by until he saw something similar to what his sister and big friend had seen. Up on an area of the wall, where a Woolly Mammoth tusk had always been, was now oddly replaced by a slightly differently-shaped tusk and labeled Steppe Mammoth.

"That's weird...," said Dipper, scratching his head. He pointed this out to Mabel and Soos, who in turn showed him the pigeon model.

"No signs of burglary," commented Dipper, "And Grunkle Stan would have announced to all of us if something had been stolen from the Mystery Shack. So what's going on here?"

"Maybe it's voodoo!" said Mabel, lowering her voice for dramatic effect.

"Or probably some really tidy burglar who cleans away their fingerprints," suggested Soos.

"Look at this!" yelped the voice Wendy, who was then seen pacing quickly over to the others. "You're all not going to believe this. While arranging the items in the gift store, I found that the Saber-Toothed Cat plush toys could have been stolen and swapped with these."

She then showed them two plush toys she brought form the store to them. Before they were representations of Smilodon but now they looked completely different in both coloration and fang size, and their tags read Xenosmilus.

"That's definitely odd," said Dipper, looking carefully at the plush toys, "Besides, Stan would be upset and be complaining by now that his merchandise is being stolen or traded."

Just at that moment, in cape Grunkle Stan, whistling to himself and then taking notice of the four young ones. "What's going on here, kids?" he asked curiously.

After the twins, Wendy and Soos showed Stan the mysterious replacements for some of the items in Shack, the old-timer decided to come up with a hypothesis to this. Suddenly, as he wondered about all this, a sort of force entered his thoughts, causing him to freeze still for a brief moment.

"What are you talking about, kids?" the old man then said after the brief freeze. "They have always been this way."

This was very odd that Grunkle Stan would be acting this way about potential thieves. It looked as if something forced him in his subconscious to say what he said. It would seem as though that the twins, Soos and Wendy were going to find out what was happening.