Author's Note: This is my first fanfiction. It's completely AU. Mark and Meredith are married and have been for three years. He is a top plastic surgeon and she is a top neurosurgeon. I hope this is okay, please tell me what I can do better or what you like. Thanks! By the way, I own no part of Grey's Anatomy.

The lyrics of I Know the Truth from the musical Aida

How have I come to this?

How did I slip and fall?

How did I throw half a lifetime away

Without any thought at all?

This should have been my time

It's over, it never began

I closed my eyes to so much for so long

And I no longer can

I try to blame it on fortune

Some kind of shift in a star

But I know the truth and it haunts me

It's flown just a little too far

I know the truth and it mocks me

I know the truth and it shocks me

I learned it just a little too late

Meredith Sloan had had a hard day at work. One of her patients had died on the table and another had died before she could perform the surgery. So her shift had ended early and the tired woman was looking forward to going home and relaxing with her husband of three years, Mark.

She dug in her purse for the keys to the brownstone on Manhattan's Upper East Side. Finding them, she opened the door, grateful to be home. She heard weird noises from upstairs and assumed that Mark was watching a movie. She kicked off her shoes, headed up the stairs and opened her bedroom door only to find her husband on top of another woman.

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Mark Sloan had always been a confident, successful man. He came from an extremely wealthy and renowned family, and during his 20s and early 30s he was New York's hottest bachelor. Sexy, charming, and rich, people expected that Mark would be a manwhore forever. And he too was perfectly happy bedding famous, beautiful women every night and kicking them out the next morning. That was until he met Meredith Grey.

She had begun as one of Mark's typical one night stands five years ago. She had actually been his former best friend's fiancée, and Mark always felt bad that he had been infatuated with her. Within a year, they had slept together and something about her captivated him: he kept coming back for more. Soon, they were sleeping exclusively with each other and somehow it became more than that. It became a relationship neither expected and a year later, with a lot of prodding from his father who wanted to see his only son married, Mark proposed. A year later they were married.

Although he had a sketchy relationship history, Mark never strayed. But Meredith was now in her third year as an attending and she was becoming busier and busier. They began to drift apart but it wasn't until that day Mark was unfaithful.

He too had lost a patient. As a top plastic surgeon, it was rare but when it did happen Mark felt the need to drink after. It was early afternoon when he walked to a local bar, had one too many drinks, and fell into his old ways, bringing a hot blonde home.

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Meredith stood in the doorway, unsure of what to do. Mark must have sensed her presence because he turned, and seeing Meredith he immediately sobered up and guilt overtook him. He rolled off the blonde, wrapped a sheet around his waist and walked towards Meredith as the other woman frantically searched for her clothes.

"Mer…" Mark started, his voice full of emotion and remorse.

"Don't," Meredith said, her voice threatening to break and the tears threatening to fall. "There's nothing you can say. I'm leaving."

"Mer, please. I'm sorry. Let me try to fix this, please give me another chance," Mark pleaded. By this time the unknown blonde had had the decency to leave the house.

Meredith shook her head sadly, "It's over Mark."

"Meredith, please don't go."

"I have to," she said, pulling a suitcase down from the closet shelf. She began throwing clothes into it, just what she needed. She was one of the top neurosurgeons in the country at the tender age of 34, and she could buy new stuff wherever she ended up.

Mark stood, paralyzed, watching her as she gathered her things. He watched as she cleared their bathroom of her personal products. He watched as she left, looked around the upstairs and returned with a few things. He watched as she closed the suitcase and dragged it downstairs. He still stood, watching the bed, hating himself for what he had done. Finally he walked down the stairs as Meredith opened the door.

He asked resignedly, "Where are you going?"

She turned and he could see the pain in her beautiful green eyes, pain he had caused. "I don't know," she answered.

"I'm sorry," Mark said, one desperate plea for her to change her mind, for her to turn around and stay.

"It's not enough."

As Meredith was climbing into a cab, Mark ran outside. "How will I find you?"

Meredith replied softly, "You won't." She shut the door and the cab drove away.

Mark stood there, still in only a sheet, as he watched her leave. He stayed for a long time staring in the direction she had gone. Then, suddenly, he turned and went into the house. He swore loudly and punched his fist into the wall.

Meredith entered the halls of Seattle Grace Hospital, the same hospital where her mother had worked when Meredith was younger. She knew that the Chief of Surgery was her mother's good friend, and one true love, and Meredith needed a job. It just so happened that he needed a neuro attending. She climbed off the elevator on the fourth floor, and ran right into a man that immediately reminded her of a man from her past.

"Meredith?" the man asked, confused and taken aback.

"Derek," she answered flatly, remembering the second to last time she had seen him, when her world changed, and when she made one of the greatest mistakes of her life.

Meredith sat on the couch, wringing her hands and glancing at the clock. She looked at the empty tequila bottle on the table next to her, and debated going to the cupboard for more. She had been drinking by herself for four hours, waiting for her fiancée Derek Shepherd to return from work. She knew residents had busy schedules, and as Derek was chief resident he worked more than others. She got it. She was studying to be a doctor, her mother was a prominent doctor, although her mother didn't believe she had what it took to be a good surgeon. Meredith understood the demands of a medical career. She understood Derek was busy, but they had purposely planned tonight to go out to dinner…over four hours ago. She didn't understand why he couldn't have at least called to let her know he would be late. Her makeup was smeared, she was drunk after pulling out the tequila four hours ago, and her gorgeous red dress was becoming wrinkled. And the doubts were beginning to creep in. Was he cheating on her? Did he no longer love her? Was this what their married life was going to be like? Was this what her life with Derek was going to be like? Just then a knock came to the door. Meredith got unsteadily to her feet, hoping it was Derek but knowing in her heart it wasn't. Derek wouldn't knock to come into the apartment they shared.

She went to the door, teetering and stumbling, to reveal none other than Derek's best, and very sexy, friend Mark Sloan. He gave her a grin dubbed by nurses and doctors alike as the 'McSteamy' look as he entered the apartment.

"Derek wanted me to check on you. He wanted me to tell you he'll be home in a few hours."

"But I don't want him home in a few hours," Meredith whined. "I wanted him home four hours ago. Tell me the truth Mark," she said sternly looking him in the eye. "Did he volunteer for this surgery?"

Mark looked down at his expensive shoes, not wanting to tell her the truth. "Yeah," he muttered finally.

Meredith's eyes began to fill with tears but she quickly blinked them back. Her green eyes flashed, and Mark could tell she was upset. But he also saw the hurt and doubt in them and then something changed. Her eyes became darker, lust-filled, hungry and passionate. Mark stepped back into the door, his back against the cool wood, as Meredith, clearly drunk, lifted her dress higher on her thigh.

Meredith stepped closer to Mark, and whispered, her hot breath tickling his cheek, "Make me forget, Mark"

Mark was a notorious manwhore but he had always been intrigued and infatuated with his best friend's fiancée. He knew that they would never be, but that had never stopped him from thinking that someday she would be the woman he settled down with. So, against the little voice in his head that told him it was wrong, Mark Sloan began passionately kissing Meredith Grey. He knew it was wrong, he knew it would hurt Derek, should he ever find out, but he also felt bad for Meredith. Derek wasn't paying as much attention to her as Mark thought he should. He did volunteer for surgery tonight, rather than spend it with his fiancée. He also didn't call, so Mark didn't feel too terrible about kissing her.

They stumbled towards the couch, feverishly undressing each other. And then, just as he was about to enter her, the door opened. Mark rolled off her, and hastily grabbed his boxers and pulled them on. Meredith pulled the blanket off the back of the couch to cover herself. Derek's face changed from shock to pure anger as he made a fist and slammed it into Mark's face. Spinning on his heel, Derek began to leave.

As he reached the door, he turned to address Meredith, " I'll get my stuff in the morning. I never want to see you again. And keep the damn ring."

Meredith was crying softly, but not as hard as Mark expected her to be. Most women would be sobbing, but Meredith had a quiet acceptance, as if she had wanted him to catch them. She was suddenly sober, but for some reason she didn't regret sleeping with Mark. He had always seemed more fun than Derek. Mark was always the one to come over and tell her Derek wouldn't be home, and Derek never had the decency to call. Derek was always too busy to spend time with her, but Mark rarely was. She turned and went into the kitchen, wrapping the blanket tighter around her body.

Mark watched after her, sure that she was planning on running after Derek. After all, Mark was not a man who had ever prided himself as being the marrying kind. His father wanted him to be, sure, but Mark was perfectly content bedding a different woman every night. But somehow, Meredith felt different. She wasn't just another notch in his headboard, so to speak, but amazingly he could see himself settling down with her. And that thought scared him almost as much as the fact that the only woman he could ever see himself in a relationship with was surely going to run after her fiancée. After all, Mark had learned in his years as being Derek's best friend, Derek wasn't the kind of man you left if you had the choice.

Mark was surprised when she returned from the kitchen with an ice pack for his face and a smile. Mark sighed. He knew he should apologize, and send her off after Derek. Contrary to popular belief, Mark was not a bad guy. Sure, he had always wanted Meredith, but he had envisioned Derek and Meredith breaking up amicably and him being able to move in without any hurt feelings. His dream had started to fade when Derek had asked Meredith to marry him, and had continued to do so as wedding plans began to materialize, but Mark still held out hope. But he worried that he would have betrayed his best friend for nothing.

Meredith knew she should feel guilty, but a part of her had always wished she had met Mark first. Derek had always been devoted to his work, he always had to be the best, whereas Mark, although just as good a surgeon as Derek, wanted a healthy balance of work and play. And while Meredith did not always approve of how Mark chose to unwind, with a string of one-night stands, she had always admired that he could stand to be away from his pager and he did turn off his cell phone once in a while. She knew she should apologize for putting him in this situation, and to tell him to go after Derek and repair the friendship, but most of her just wanted to pull his sexy body back down to the couch and continue where they had left off.

And this was, much to Mark's surprise and happiness, exactly what she did.

And now, here she was, facing the man she could have had a life with. But she had thrown it away for another man, a man who less than five years later would cheat on her and break her heart.

Derek broke the awkward silence as he asked, "What are you doing here?"

Meredith straightened. "I'm the new neuro attending."

She saw Derek's eyes stray to her left hand, where her rings lay, the rings she had not had the heart to take off yet. She hadn't been able to accept that her marriage was over, that she had left her fiancée only to have a broken marriage with another man.

"You're married," he said, more of a statement than as a question. "Who?"

"Mark," Meredith replied calmly.

Derek's eyes immediately darkened and his handsome features became angry. "Mark? You married that bastard?" he said accusatorily.

Meredith bristled. Although Mark would soon be her ex-husband, she still felt the need to defend him because, although she would not admit it to herself, she was still totally and completely in love with him. And besides, Derek had had almost five years to get over their betrayal.

"Yes, I married him. And it is none of your business." She remembered the last time she had seen Derek, when he tried to practically force her to marry him a month after he caught her with Mark. He didn't like failure, and a failed engagement was something he would not let himself admit to. But Meredith had seen the hate in his eyes, and the disgust and revulsion he so obviously felt when he looked at her, that she had refused. Even when he hurled insults, threatened her, told her she would never find anyone else, and that he never wanted to see her again, she stood her ground. She only cried in the arms of her lover, Mark, as by now they were sleeping exclusively with each other.

He eyed her clothes, her street clothes, because she was on her way to the locker room. Meredith saw this and bitingly returned that she was going to change and left him, standing there in the hallway, fuming at his run in with a woman he never thought he would see again.

So there's Chapter One. I hope you all liked it. I'll post more if people respond to it. Thanks!