A/N: This isn't really a new chapter, just an exploration of Sheldon's possible motivations during the last one...

"Ok, sweetie. I'm gonna go, cause I don't wanna make this any worse for you. Ok?"

Sheldon felt Penny get up from the bed. His bed, that she had just came in and sat down on. He tried to remember why he hadn't objected.

The movement sent a waft of something citrus scented in his direction. It reminded him of Christmas; the Leonard Nimoy napkin and the baskets of bath stuff he had given her. It reminded him of their shared hug. He realised suddenly that she had been trying to help him and he had been so wrapped up in his own pain that he had responded… inappropriately. Yet again.

"Penny?"

He made it off the bed as she was reaching the door. She looked at him expectantly. "Thank you. For trying to make me feel better." He had a brief moment of doubt, but saw the smile flash across her face and gingerly put his arms around her. He felt her hands on his back; motionless at first as she waited for a reaction from him, then moving soothingly as she realised he wasn't pulling away. He knew the cavernous pain in his chest was caused by emotion; a psychosomatic reaction to his distress. He didn't know why hugging Penny made the pain recede. He gripped her harder, pulling her to him, and felt it melt away to a dull memory.

He felt her relax against him, and abruptly became aware of how close she was. Her breasts were pushing against his chest and he felt something unfamiliar stir inside him. Then his hand was burying itself in her hair, seemingly of its own accord, and she still wasn't pulling away. He tried to analyse the situation, interpret it, but he realised he had become a victim of his own primitive biological urges.

He wanted her.

What was more incredible was that she seemed to want him too. She was breathing harder now, and he hovered uncertainly for a moment, considering whether to take the next step. A small voice in his head urged him to think of the consequences, and he debated himself for a second before resolutely ignoring it. Then his apparently autonomous hand was stroking her hair away from her neck, exposing it invitingly, and he pressed his lips to the bare flesh. Penny made a small noise deep in her throat and any remaining reservations vanished.

Everything happened quickly after that. Her hands were in his hair, directing him, and when she moved herself against him he lost the capacity for rational thought. He gave himself over to his urges, and revelled in the responses they elicited from the beautiful woman beneath him. It was only when he was inside her that he seemingly returned to himself; the incredible feeling of being immersed completely. He froze, unable to comprehend how they had got here, and the thought suddenly hit him; this was what she was going to do with Leonard.

"Oh God, Penny…"

Leonard, the man who had orchestrated his humiliation.

He didn't feel sad anymore. He felt angry.

"Sheldon. Fuck me. Now."

The words made him feel powerful and terrible at the same time. He would probably never get to be with Penny like this again, and it was because of Leonard.

He could feel Penny moving beneath him, urging him on. Suddenly it didn't matter. Nothing mattered.

He fucked her with everything he had, pouring all of his hurt and frustration and humiliation into the act. She bit him hard when she came and he relished the pain, letting it drive him on. Then he was exploding inside of her, the intensity of the feeling overwhelming him as he cried out.

Afterwards, he felt spent. The surge of emotions he had felt had ebbed away; he was left with confusion. He watched Penny as she caught her breath, a crushing feeling of tenderness flooding him. He understood now how he thought Leonard must feel towards her, and he suddenly felt guilty. He examined his motives for seducing Penny (not that she had put up any resistance, his psyche reminded him) but found them honourable. Revenge had not played a part. He mentally apologised to Leonard; he was sure there was a social convention that dictated you should not sleep with your best friend's girlfriend immediately before the successful culmination of two years worth of seduction.

(Maybe, but he deserved it)

He was running through these thoughts when he heard the front door open.

"Penny?!" he heard Leonard shouting. Something niggled at Sheldon's brain. He watched as Penny jumped up, started panicking. He slowly started pulling on his clothes, distracted.

"Penny? You in there?"

There it was again. Something was irritating him about the situation. He watched as Penny opened the door to Leonard.

Who completely ignored him.

Sheldon suddenly realized what was pissing him off.

The last time Leonard had seen him, he had been upset and stormed out of Penny's apartment. Now, here he was, in Sheldon's doorway, looking for Penny. He hadn't come over because he was worried, or wanted to apologize. He had come because he wanted to find Penny, no doubt with the intention to pick up where they left off.

Sheldon felt the rage building inside him again. Leonard – the man who had hurt and humiliated him and who clearly didn't give a damn - had come here for Penny. He didn't know which aspect of that thought made him feel worse, but watching her leave, he felt a pang of jealousy.

He looked at Leonard, who was clearly still not concerned with anything other than getting laid.

"Erm, what happened there?" He looked confused. Sheldon shrugged, felt a twitch of pain in his shoulder, and watched as Leonard's face gradually changed.

Oh. Interesting…

"Sheldon, where did you get that bite mark?" The look on Leonard's face was almost comical.

He weighed the odds of ever getting to be with Penny again, and found them slim. Well, if he couldn't have her…

Ignoring the voice screaming at him in his head (if you do this nothing will ever be the same again) he made a decision.

He might not have slept with Penny for revenge, but by God he could use it.

He felt a rush of exhilaration.

"Oh, that?" he paused. "That was from Penny. What was the phrase she used, let me see…"

He stood close and bent down, using his height to its full potential, determined to go for maximum impact.

"Oh yes, I believe it was 'Sheldon. Fuck me. Now.'" Speaking her words, he felt a heady combination of delight and terror at the destruction of the man who had destroyed him.

"Did you?"

Leonard was deathly pale, but Sheldon felt unable to stop even if he wanted to. He paused, for maximum devastating effect, then narrowed his eyes and sighed.

"Oh Leonard. Of course I did."

The feeling was exquisite.

He went to leave, moving past Leonard who was slumped against the door frame looking sick, then had a thought and stopped. He eyed his best friend, remembering the casual dismissal of his hurt and anger, and chose his words.

"But hey. Don't worry about it. I mean, it's not a big deal. Right?"

He smiled.

And tried not to think about Penny's reaction when she found out.

A/N: Ok, anyone think I should have made him totally evil and had him plan it all from the start? I was considering it. I might just write it for fun…