Three times Penny dresses up.

:His Particular Poison:

His mouth was dry and his palms were sweaty. His knees were wobbly and his stomach was churning. His pulse pounded in his ears. His eyes narrowed as he watched her.

The green tights were opaque. The bodysuit clung tightly. Vines twisted around her left leg, across her stomach, and curled around one breast. Leaves decorated her crimson wig. Her green lips glistened under large emerald eyes.

A guy in a Captain America costume approached her with a leering grin. He watched her cock her hip out and flip her hair back as she continued to speak with Howard and Bernadette in their Ash Ketchum and Misty costumes. Captain America reached out and caressed her arm.

He didn't even realize he was moving until he was pressed right up against her back, glaring at the Avenger. He slid an arm around her waist and she smiled up at him enticingly. The guy held up his hands in surrender and backed away with a rueful grin.

"Moon Pie, Spock isn't supposed to get jealous," she laughed gently.

Sheldon looked down into her smiling eyes. "Spock never had a woman like Poison Ivy to watch over."

Penny laughed and placed a tender kiss on his lips, making him blush happily.

:Mario Party:

Sheldon brushed his mustache carefully and huffed impatiently. He smoothed his Luigi cap and looked around. Leonard was adjusting his Mario suspenders. Howard was using his King Koopa claws to playfully menace Bernadette and Amy in their Princess Peach and Princess Daisy gowns. Raj was trying to keep his Toadstool hat from falling over his eyes.

Just as he was about to begin complaining, the lobby doors opened and Penny came out. Sheldon's eyes narrowed and his face turned slightly red. Penny was wearing a trench coat and wide brimmed hat.

"Penny! You are supposed to be Holly Koopa! Not Carmen Sandiego!" he exclaimed. Penny rolled her eyes and headed to Bernadette's car. She slipped into the back without a word.

Sheldon moved closer. "Penny, did you hear me? I asked you.." Penny placed her hand over his mouth and turned to Leonard.

"Can we go please?" she asked.

Sheldon jerked away from her and stormed away. "She is ruining everything!" he muttered. He climbed in the car and buckled up. Then he crossed his arms angrily and ignored everyone.

Soon the two cars pulled up at Cal-Tech for the costume party. The moment the cars stopped, Sheldon scrambled out and hurried to the entrance, refusing to glance at Penny. He kept his eyes forward as they registered for the costume contest. In fact, it wasn't until Kripke dropped his drink and Winkle stopped talking and gawked, that he turned around. Penny had shed the trench coat and wide brimmed hat. Sheldon's brain shut down.

Her hair was pilled into a ponytail on top of her head. It was threaded through a small blue pillbox hat. The tip of her ponytail was dyed red. She was wearing a form-hugging black cat suit. On her back was a large orange windup key.

Penny smiled at something Amy said and his brain sputtered back to life. Kripke walked over and leered.

"Woxanne is da bomb," he said with a smirk.

Sheldon shook his head. "No, she is the Bob-omb," he replied. Sheldon felt a shiver run down his back as he finally acknowledged to himself that where Penny was concerned, he was no Homo Novus after all.

:Her Doppler Effect:

(Thursday)

The door opened and the guys looked up to see Penny walk in. She walked over and held out a sheet of paper to Sheldon. He took it and scanned it quickly. He looked up at Penny and then read it a second time.

"I will want this witnessed and notarized, you understand," he said with a touch of glee.

Penny nodded. "Yep. I promise not to call you by your Meemaw's nickname for 6 months, and you have a free pass to clean my apartment whenever you see fit for one year." The other three gasped and stared at her. "In exchange, I get to borrow your Doppler costume and your Flash costume for the weekend. I also promise to have them dry cleaned and returned unharmed by next Friday."

The guys looked between her and Sheldon warily.

Sheldon narrowed his eyes. "Why?"

"I'm going to stay with a group of friends in Vegas this weekend. We have decided to make it a costume event. Each night has a theme. Friday is sports day. We wear our favorite team jerseys and we can only speak by yelling, as if we were at a game. Saturday is gender pop culture night. Everyone goes as a popular pop culture icon, only in the opposite gender. I'll go as a female Flash. Sunday is non-human night. Most will go as aliens or, say, salt and pepper shakers. I'll go as the Doppler Effect."

Sheldon stared at the paper for a couple of moments. "Deal," he said. He had Raj and Howard sign the paper as witnesses and then stamped it. He walked back to his room and returned with two garment bags. Penny thanked him and left.

(Sunday)

The guys were on the fifth Star Trek movie when the door burst open. Penny walked in and the reactions were immediate. Howard groaned and slid off the arm of the couch. Raj spit his water all over his lap. Leonard dropped his fork. Sheldon swallowed hard and turned a bright red.

Penny's hair was slicked back into a tight bun. Her lips were black with thin white stripes to match the costume. Her eyes were smoky. Sheldon's costume fit. Barely. The pants were tight enough to show the line of her thong. The top was tight enough to show she wasn't wearing a bra. Penny moved across the room and leaned down to place a quick kiss on Sheldon's lips.

"Thank you sweetie! I won first place last night and tonight! I'll drop your costumes off in the morning at the dry cleaners." She waved at the others and then left.

Slowly Howard looked over at the others. "Wow," he said with a wiggle of his eyebrows. "Who knew something that was neither sound nor light could be so sexy!"

Raj and Leonard nodded and added their own agreements. Silently Sheldon agreed, too. Very sexy indeed.