Pinkie Pie and Twilight walked through Ponyville at a much more cautious pace than normal. Cautious, because the entire town was shrouded in a deep fog. "Have you EVER seen fog so dense before, Pinkie?" Twilight asked.
"Nope! Can't say I have!" Pinkie replied with a hop, which Twilight couldn't see. "This fog is as thick as cake batter!" Pinkie said, then started on another long-winded tirade about the thickness of the fog.
Twilight wasn't willing to admit anything was too mysterious about this fog, and, leaving Pinkie to her talking, walked a few yards away and focused on a fog-splitting spell. She didn't worry as her horn lit up. She'd know where Pinkie was.
The area around her horn lit up, and did little besides that. Twilight focused, but winced as Pinkie's talking interrupted. Focusing again, Twilight attempted to create a beam of light. The light shone, but not nearly far enough to split the fog.
Grunting in frustration, Twilight rejoined Pinkie by following her voice. "Pinkie," Twilight asked, both for wanting an answer and also to change the subject for her rambling friend, "how can you see where your going?"
Pinkie stopped, and Twilight bumped into her. "Well, DUH!" Pinkie answered, "I just use my PINKIE SENSE! I can tell I don't want to go that way or that way," if Pinkie was pointing, Twilight couldn't see which way, "so I go THIS way!" Twilight gasped, not knowing where Pinkie Pie had gone. Her voice was growing more and more distant.
"Pinkie, Pinkie, come back, I-" Twilight bumped into something. Didn't feel like a pony. Putting a hoof out, Twilight found it was a building before her.
"Pinkie. PINKIE!" Twilight called, to no avail. Trying to move around it, using a hoof as a guide, Twilight stepped to the left of the building. More building. More building. More building.
Twilight moved her hoof up and down as she walked, then suddenly found the top of the building. Twilight stopped. The top?
Backtracking, standing on her back hooves, she found the building was too high to reach for, but further down it was no taller than her. A gate maybe? Twilight didn't recall any structures like this in Ponyville. Twilight groaned with annoyance, then hopped over the wall. As soon as her hooves touched the top, she toppled over. Rocks met her face as she landed. Twilight sat up, rubbing her face, then kept going. She was angry at Pinkie Pie for not coming back to help her. But, then again, maybe Pinkie forgot every so often that no pony but her had the navigational skills she did.
Twilight walked on, feeling the land shift under her hooves, which was accompanied by a cool gust of air. Now Twilight was concerned. "Where AM I?" Twilight couldn't begin to guess where Pinkie had taken them. Must not be any place dangerous, though.
Twilight realized she spoke too soon when her forehoof left the ground and couldn't find anything to step back on. After a brief attempt to counterbalance, Twilight fell. She didn't fall far, nor hard, but she was annoyed none the less. None of her fog-penetration spells worked and she couldn't begin to guess where she was!
Twilight turned around, wondering where Pinkie was, then stopped, gasping. The fog had cleared just enough for her to see a cave before her. From it came the milky mist that accumulated into the fog around her.
"Pinkie?" Twilight asked, not expecting much of an answer. She didn't receive one. Once inside the cave, Twilight tentatively strode down it's cold stoney maw as she attempted to shed some light about her. Finally, with enough concentration, she broke through the tiny strands of fog and could see before her. She could see her own front hooves, but not much else.
That satisfied Twilight, though, so at least she knew she wouldn't walk into any walls. "PInkie?" Twilight called, her voice echoing down the cave. "Pinkie, if this is a joke, it's getting old," Twilight hung her head, "REALLY fast."
Twilight started to worry, though, as she walked for a few more minutes, if she would ever know where she was. Or how long it'd be before she could get home. Or if Pinkie....
Twilight smacked herself with a hoof. No, it was impossible. Pinkie would let her know if she was hurt. They just made a wrong turn somewhere.
However, Twilight started to worry more and more as the tunnel kept growing longer and longer. Finally, she couldn't take it anymore and ran, deeper and deeper into the cave. Even though she'd just started running, she breathed heavily. She felt herself getting dizzy. Her lungs were pounding. There was no air! Where was the air?!
Right when Twilight reached the breaking point, she felt a gust of air on her face. With a throaty laugh, Twilight fell to her knees, got up shakily, then hobbled out towards what looked like an exit. Twilight wasn't sure if she was happy or sad or relieved or all three when she broke out of the cave into the night. Pausing for a moment, Twilight caught her breath, calming herself.