A 3-year-old Mark Sloan walked into the room alone, his mother having driven off as soon as he pushed the car door shut. A tall woman approached him, her heels making a clacking sound on the hard linoleum, that instantly made him cringe as if the sound caused a repressed memory to resurface. He kept his frown on his face as she smiled too widely at him, making him promises of fun.

He stomped on her shoe and walked over to a table where a few other children were coloring. He sneered at them, in his all-black outfit, complete with leather jacket, and demanded they give him their crayons. They did so, fear in their eyes as they ran from the table. One little boy ran to the teacher, crying about the mean new boy.

A peaceful expression at last fell upon his face as he sat in solitude, his paper and crayons his only companions. His isolation didn't last long, though. Before he knew it, a group of girls were making their way over to his table. They were quite impressed by the new rebel.

A 3-year-old Derek Shepherd walked into the room, clutching his mother's hand tightly in his right hand, a doctor's bag given to him by his father in his left. The pair made their way over to the teacher, who was still consoling the little boy from the coloring table. She smiled at them and sent the little boy off to play, giving Derek and his mother her full attention.

The teacher made Derek the same promises of fun she had made Mark and a smile came across her face once she realized Derek's incessant stomping didn't mean he was going to step on her feet. He just had to go to the bathroom.

He ran off, while his mother continued to talk to the teacher. He came back moments later to ask her if she wanted to play with him. She bent down to his level and told him, as gently as she could, that she had to go grocery shopping and his father would come to pick him up later. He nodded, trying to be the big boy his daddy had told him he was. She left, after a long hug, and he turned back to his teacher, only to find she had moved on.

He surveyed the room and his attention was caught by a little boy who was scribbling intently on a piece of paper, trying to ignore the bevy of girls surrounding him. One tried to borrow a crayon and he got up to storm off, only to have them chase after him. Derek quickly assessed the situation and dug through his doctor bag until he found what he was looking for. He held the instrument proudly in the air, waving it in the air as he ran up to the boy, yelling, "Follow me!"

The pair ran and hid behind a cubby, safe from harm's way. "Girls are icky!" exclaimed Mark.

Derek nodded enthusiastically. "Super icky! Do you want a cootie shot?"

Mark stared, wide-eyed, at the boy in front of him. "A cootie shot?"

"To save you from all the girl cooties!"

Mark nodded enthusiastically, keeping a brave face on as Derek gave him the pretend shot. "Thanks. What's your name?"

"Derek."

"I'm Mark."

The boys played together throughout the day and waited anxiously as the parents began to show up. "My daddy's a surgeon. He prolly got stuck in surgery," Derek rationalized her father's lateness proudly. The clock kept on ticking until Derek and Mark were the only two children left. Derek's mother ran the room, her face displaying obvious relief as she spotted her son.

"I'm so sorry, sweetie. Daddy had to work late. And I was cooking dinner and ... oh! I forgot to turn to stove off!" She quickly got up to call the neighbor to tell her to turn the stove off. She returned and looked down at Derek expectantly. "We're going to have to get pizza tonight. Ready to go?"

Derek stole a quick glance at his new friend before turning back to his mother and shaking his head. "I can't leave Mark."

"Oh, well sweetie, I'm sure his mommy will be here soon. And Nancy got done with swimming 20 minutes ago so we really need to go pick her up." She turned to Mark, asking, "Your mother will be here soon, right?"

He merely shrugged his shoulders as he sat with his knees to his chest, his arms resting on his knees, his head resting in his hands. She suppressed a sigh before pulling up a chair to sit by the two boys. The teacher said it would be fine to leave and that she would stay until Mark got picked up, but Derek's mother just shook her head. "My son is not leaving until his friend gets picked up."

Another 15 minutes passed before the preschool phone rang. Mark's mother was calling to say she had lost track of time and she was running late. Derek's mother grabbed the phone from the teacher and asked how long she was planning to be. When she said she was 45 minutes away, Mrs. Shepherd shook her head and said she would take Mark home with her and his mother could come pick him up from there. She quickly gave her the address and hung up, loading the boys in the car as she sped to pick up Nancy before heading to the local pizza parlor.

The next morning, she took both boys back to preschool, Mark's mother never having shown up to pick him up.

As the years passed, Mark became a semi-permanent fixture in the Shepherd household. He mother was frequently absent and his father had disappeared a few weeks before his 4th birthday. Mrs. Shepherd was happy to take him in and adopt him as one of her own, even as her own family kept expanding. Kathleen was born when the boys were 4; Beth and Michelle when they were 6.

Derek and Mark constantly looked to each other for salvation as their home became more and more invaded with females and Derek's dad became more and more absent. They relentlessly begged Mrs. Shepherd for a tree house, and when they finally got it, they made a sign declaring 'No Girls Allowed' and used it as their constant hide-out.

They kept up this game until middle school, when they each started realizing girls weren't that bad. Naturally, their attention was both drawn to the same girl. Addison Montgomery. Mark was the more aggressive of the two and had pursued her first. She had rejected him, though. Derek had stepped up, eager to play the valiant gentleman and win out over his friend. They had "gone out" for a week or so before he had gotten into a fight with Mark, much to Samantha's excitement. They hadn't talked for a month afterwards.

They had made up eventually, though Derek claims it was only because of Beth's whining about missing her "other brother" and made it to high school, friendship intact.

During their high school years, Derek and Mark played the field quite a bit. Neither had a girlfriend for more than two weeks, up until their senior year. That was the year Derek's father, Michael, had passed away due to complications from a stress-induced heart attack. Being a doctor had literally killed him.

Mark was so sobered by the news that he kept a girlfriend for several months. Her name was Isobel Stevens and she had constantly supplied the family with home-cooked meals, offering her sympathies every 2.3 seconds.

Eventually, though, she became too annoyingly perky for Derek and he was quick to snap Mark out of his funk, making both men single once more.

The family was distraught for months until Derek and Mark both stepped up to the plate, taking the position of 'men of the house', Mark having, for all intensive purposes, become a Shepherd. They continued fulfilling their self-imposed duties even after they went away to college, and later, med school.

They were there to interrogate Kathleen's first major boyfriend, George. George was a small kid with akward hair who seemed much too nervous for a high-school boy, but he came across as nice and sincere, so he was given the stamp of approval by the two men.

Kathleen and George dated for a year and a half before George met Izzie, who had stopped by the Shepherd household on one of her frequent visits home from school. She was under the illusion that she and Mark were still friends, even though Mark had managed to be out of the house for the past 27 times she had tried to visit him.

Still, she didn't get the message, and she tried once more. George answered the door, having become somewhat comfortable in the Shepherd household. The two began chatting while Kathleen finished getting ready for her date with George. When Kathleen came down, they said good-bye, but exchanged numbers to stay in touch. The mousy boy had fixed his hair and was more confident.

Mark and Derek were at med school at Johns Hopkins when they got wind of the news that George had cheated on Kathleen with Izzie. Immediately, they scheduled a trip back home to Maine. They showed up on George's doorstep two days later, much to his surprise, and fulfilled the promise they had made him about what they would do if he ever hurt their sister.