Twilight woke up to find a mare in her bed. A mare that wasn't Luna. 'What happened last night!?' Twilight thought to herself. She looked closer at the mare and saw that she had a black coat and a purple/blue mane colour scheme. She held her head and groaned slightly.

“Hangover. Must have been drinking a lot last night. I got separated then everything else became a blur.” Twilight muttered to herself. She took her hoof away and slowly climbed out of bed. Leaving the mare there she tiptoed across the room and opened the door. Twilight closed the door behind her and let out a sigh. Peeking over the balcony that overlooked her library, she could see that a Pinkie Party had taken place. A very heavy-drink-em-up Pinkie Party. There seemed to be a few ponies lying on the floor, out cold. A couple were draped over the railing and a few more were sleeping on the stairs. 'What happened here?' Twilight thought.
Shaking her head to try and clear it, she went to the bathroom and splashed some water on her face. It helped but she still felt that she was intoxicated. Walking out, she felt woozy and thought to sit down for a bit. When she did sit down, a blue aura of magic enveloped her and transported her.

“What fresh hell is this?” Twilight winced as some harsh light was given to her retinas. A midnight blue mare swam into her vision. “Princess Luna? Where am I?” Twilight asked. She didn't have any clue as to her whereabouts because all that she could see was a harsh light playing off of Luna's coat. Everything else inside her line of vision was pitch black.

“You are in a place accessible only to me.” Princess Luna sounded vague. Switching tact, Twilight decided on a different approach.
“Okay. Then why am I here?” Twilight asked, hoping to get a different answer.

“You are here because you cheated on me!” Princess Luna nearly went into her Royal Canterlot Voice but held herself in check. The Princess of the Night began to walk around the rope-bound mare. Twilight tried to follow her by turning her head but found it locked in place by ropes. Her hooves were bound just as tightly.

“Ch-cheat? Why w-would I ch-cheat on you?” Twilight stuttered nervously. Princess Luna stopped behind Twilight's head.

“I don't know. Why would you cheat? Hmmm? Let me try and make it more clear, seeming you are confused about the issue. Last night perhaps? Pinkie threw us a random party? Starting to get your memory back?” Luna asked condescendingly. Twilight couldn't move as she struggled against the ropes. She then thought about her magic, and tried to untie herself that way. She concentrated and all she got was few fizzles. “I put a dampener on your horn. Didn't want you running away.” Luna said.

“Last night? Last night, Pinkie threw us a random party, yes, but by the time we were intoxicated we had split up. I thought you went home. I then found a random mare and slept with her instead.” Twilight gasped, finally cracking under the pressure.

“Home? Twilight, I live with you.” Luna pouted.

“Yes, but you were needed at the castle for something or other.” Twilight said flippantly. Luna just chuckled and resumed walking around her captive. “Think you can let me go now?” Twilight asked hopefully.

“Why would I do that dear? You sill haven't had your punishment.” Luna said with an evil glint as she stopped walking. Twilight could just see Luna's flank in the corner of her eye.