Sheldon Cooper was enjoying a rare solitary night in his apartment when a soft, tentative knocking caught his attention. He strode to the apartment door and opened it to find Penny standing there, a few wrapped packages in her hands. He quirked an eyebrow at her unusual behavior. Although she had been living across the hall from them for several months, their pretty blonde neighbor was already in the habit of barging in without knocking. He acted as if he was annoyed by it, but he too had grown up in a small southern town where company knocked, but friends just walked right in.

"Hey, Sheldon," she said uncertainly. She craned her neck, trying to peer over his shoulder. With Leonard, this would have worked, but Sheldon was a full six inches taller than his roommate. Finally, she gave up and asked outright, "Is Leonard here?"

"No, his sister was slated to receive some award, so his mother persuaded him to fly home for the occasion," he answered, stepping aside to let Penny enter.

She stopped short in the living room, looking around her in dismay. "There aren't any decorations. Why aren't you ready for Christmas?"

"I don't believe in celebrating the ancient pagan festival of Saturnalia."

"Huh?" Penny asked in confusion.

"In the pre-Christian era, as the winter solstice approached and the plants died, pagans brought evergreen boughs into their homes as an act of sympathetic magic, intended to guard the life essences of the plants until spring. This custom was later appropriated by Northern Europeans and eventually it becomes the so-called Christmas tree."

"Um, okay. I just brought over a couple of little presents for everyone," she said. She set a couple of small, brightly-wrapped packages down on the coffee table. "This one's for Raj: I got him swing dancing lessons with this online deal; he's gonna love it. I got Leonard a travel mug with that swirly atom thing on it, and for Howard, I got him a book: 101 Worst Pick-up Lines. I figure there's at least a few of them in there he hasn't tried yet," she said with a wry grimace.

"I see. I've noticed that you haven't mentioned getting a gift for me, which is good, because I-"

"Didn't get anything for me? That's okay. I wasn't expecting you too. Besides, your gift isn't something I could wrap anyway." Penny held up her phone and pressed a few buttons. "There!" She held it out triumphantly.

From Penny's phone came a voice Sheldon instantly recognized: "Merry Christmas, Sheldon, from your friend Bill Shatner. And just so we're clear, this does not mean you can call me at my house."

His jaw dropped. "That was William Shatner, the man who brought the beloved character of Captain James Tiberius Kirk to life in Star Trek!"

Penny grinned broadly. "I know! Of course, I know who he is."

"But how did you meet him and get him to record that message? And why wasn't that Leonard's present?"

"Well, one of the waiters at the Cheesecake Factory also does catering events on the side. His boss was looking for extra help for a party, and I needed the money, so I went along. Turns out the party was at Shatner's house. He was really nice, and he wanted to tip all the wait staff extra, but by the time he got to me, he was a little short on cash. So I told him I'd take a message for my friend instead. I already bought gifts for everyone else, but I couldn't figure out what I should get you. Nothing seemed quite right, so…" She trailed off with a shrug.

Sheldon stared at her for a moment, and then his face clouded over. "This is terrible!" he burst out irritably. "This is exactly why I hate the tradition of exchanging gifts. Not only am I faced with the quandary of attempting to estimate how much a certain person might decide to spend, I often find that I have either overspent or worse, I am in their debt. Now you've given me a present without monetary value. How on earth am I supposed to accurately calculate its worth?"

Penny scrunched up her nose. "I was trying to do something nice for you," she protested. "Don't you like it?"

"Like it? It's the best gift I've received in the last ten years. So now what am I going to do?"

Penny sighed and looked away. Just then, she noticed a small bunch of leaves hanging over the doorway. She glanced back at Sheldon, and her lips quirked upward in a devious grin. Walking over to the doorway, she positioned herself directly under the mistletoe. "I can think of one thing you can give me that doesn't cost money," she drawled.

"What are you talking about?" he snapped.

She rolled her eyes upward. He still just looked confused, so she nodded her head toward the greenery.

"Blast it, I told Leonard that under no circumstances was he to hang up that absurd pagan symbol of fertility!" he cried. "Doubtless he was attempting to entrap you or any other unsuspecting female into a physical act of dubious consent: the proverbial stolen kiss." Then Penny's gesture seemed to register, and his expression turned truculent. "I don't celebrate either Christmas or ancient pagan rituals," he muttered, glowering at the mistletoe.

"Oh, well in that case, I guess that means you're in my debt," Penny said casually, pretending to study her manicured nails. "Ooh, this is gonna be fun. I wonder how many favors that comes out to? Maybe you can give me a pedicure." She had no doubt he would be good at it but also knew how much he would hate the idea.

"Oh, good Lord," Sheldon muttered with a panicked look on his face. He seemed to realize he was trapped between a rock and a hard place.

"Have you ever kissed a girl, sweetie?" Penny asked gently.

"Of course not! I don't have time for such ridiculous pursuits! I'm a man of science," he proclaimed.

"Well, then, consider this an experiment," Penny said. She was no longer smiling. In fact, her eyes darkened as she watched him.

With a sigh, he walked toward her. "I suppose this is an appropriate reciprocal gift," he muttered.

"Only if you make it good," she purred.

He looked startled at that, but nervously put his hands on her waist. He stared down at her, closer than he had ever been. She smiled invitingly and linked her hands behind his neck. With a deep breath, he bent over her, closing his eyes as he drew close. He missed.

"Sheldon! That doesn't count. You only kissed my cheek!" Penny cried. Before he could react, she pulled his head down to hers and planted her lips on his. When she finally released him, she said, "There, that's how it's done."

"At least it is done," he complained aloud, trying desperately to make sense of what he just happened. His heart was pounding, and he could still taste her cherry-flavored lip gloss. Without thinking about what he was doing, he ran the tip of his tongue over his lips.

She watched him with a little smirk. "Oh no, I just kissed you, but I'm still waiting for you to kiss me."

His jaw dropped. "Penny…" he said weakly. "That's not fair."

She merely raised one eyebrow at him, and he groaned at the gesture. Every time she did that, it reminded him of Spock, which made her seem mysterious and alluring. He found himself pulling her close again without even thinking about it. He pushed every conscious thought out of his head as he kissed her. He couldn't resist opening his mouth to sample the cherry sweetness of her lips. She nipped at his lower lip, an action that sent a jolt of sensation straight down to his groin. His hands tightened on her waist, willing her to do it again, and she obliged, this time following it up with a teasing swipe of her tongue. He gripped her hips, pulling her hard against him. Her tongue darted into his mouth, and he reciprocated. He didn't notice he was edging her backward until they collided with the door. Penny hooked a leg up over his hip as she moaned and arched against him. The movement was exquisite torture as she ground against him. He only pulled away when she started tugging at the hem of his undershirt as if she wanted to remove it.

He stepped back, panting and out of breath as if he had just run for blocks. Her hair was disheveled, and her lips were full and red. Seeing her like that had him fighting the urge to drag her back to his bedroom and find out firsthand what coitus was all about. He swallowed hard, desperately trying to reign in the primitive impulses that were usually under such tight control.

"I suppose we're even now," he said, and his inflection made it half a question.

"Even, yes. Finished? Not even close," she answered as she wound her arms around his neck and pulled his head down to hers once more.

By the time Leonard got back from New Jersey, Sheldon and Penny were officially a couple. Within two days, he learned he needed to order noise-canceling headphones. He'd been upset with Sheldon at first, but eventually, he had to admit that what he had felt for Penny was just an infatuation. His roommate and his next-door neighbor were surprisingly good for each other. With Sheldon's guidance, Penny made a renewed effort in her acting career and soon had a regularly recurring role in a TV show. She helped him as well to learn how to be civil to his sponsors, superiors and coworkers. When Sheldon was awarded a grant to conduct experiments on monopoles near the North Pole, he was surprised at how reluctant he was to leave Penny, even for the opportunity of a lifetime.

He returned three months later with terabytes of data that proved absolutely nothing. The moment he saw Penny again, he went down on one knee in front of her and proposed. He told her his failed experiment was the best thing to ever happen to him, since those months away made him realize he never wanted to be apart from her again. They were married on Christmas Eve at his Meemaw's farmhouse. There was mistletoe hung everywhere, and they celebrated their happiness under each and every one.

A/N: Thanks to Delighted Guest for pointing out that little oops. It was just a juxtaposition, but I definitely wouldn't want to be lynched by angry Star Trek fans!