A/N: Thank you so much to BeautifullyTragic6 for making the trailer that inspired this fic and for allowing me to post this story. It does not do justice to her amazing video: Sherlock Series 4 Promo #4: "Detective VS Assassin", and her amazing video editing skills. This chapter marks seven days until Sherlock Series 4. SERIES 4 SPOILER WARNING: This story does use the confirmed name of John and Mary's baby.

Chapter 1:

It had been a year since John Watson had forgiven his wife Mary for lying to him about her past life as an assassin. The Watsons had made up just in time to start preparing to become parents for the first time. They chose to name their baby daughter Rosamund Mary Watson, Rosie for short.

John struggled to hold onto the screaming baby as Mary gathered all of the things that Sherlock would need to take care of her before he arrived. He and Molly were babysitting for the day because both John and Mary had to work. Usually Mary and John would try to alternate days, but Mary was called in to work at the last minute.

There was a knock at the door just as John handed Rosie off to Mary. John answered the door while Mary set Rosie down in her play pen.

"Hi, John," said Molly with a cheery smile on her face. She waved at Rosie who was now sitting calmly and happily.

"Hello, come in," John said. He opened the door allowing Molly to enter. She was followed by Sherlock who did not look happy to be there.

"Lighten up, Sherlock," said Mary. "I thought you were starting to get attached to Rosie."

"I was busy with a case," Sherlock replied quickly.

"I'm not John. I know when you're fibbing," Mary said with a knowing smile.

"Don't worry, Mary. He's happy to be here, even if he won't admit it," said Molly.

Molly walked over to see the baby. "Hello, Rosie. Do you remember me? Aunty Molly?" Rosie laughed, reaching up to grab on of Molly's hair that hung over the edge of the pen.

"Well, we should be going. I'm sure you guys have everything under control," said John. This was the third time that Sherlock and Molly had come over to babysit.

"All of Rosie's stuff is right here," Mary said, pointing to a care bag she had put together. "I just have to check on something. Back in a second." She quickly left the room to go upstairs.

"Thanks again for doing this, Molly," John said.

"It's no problem. I wanted to," she said. "Besides, I'll make sure that Sherlock is kept busy this time."

"Alright, shall we go then?" said Mary as she came back downstairs.

John led the way out the door. "We'll be back before six. Take care of him," Mary said, winking at Molly.

"I will," Molly said, smiling back at Mary. "Say bye bye, Rosie."

Once John and Mary were gone, Molly turned to Sherlock who was sitting on the couch, checking his phone. "If we have until 6:00, that means we're here for a full…" She glanced at the clock in the kitchen. 8:00. "…ten hours."

"Bored. I could solve 3 cases in that time," Sherlock said.

"Well, you aren't going to. You are going to help me take care of Rosie," Molly said.

"Fine," Sherlock sighed. He jumped up from the couch and walked to where Molly was standing.

They both spent the next couple hours playing with the baby, feeding her and changing her diapers. After Sherlock and Molly had made lunch for themselves and Rosie had settled down for a nap, they both sat down to take a break. Sherlock sat back down on the couch where he had been sitting before, and Molly took a seat in the armchair to the right of him.

Eventually, Molly tucked her feet up on the chair and fell asleep. Once Sherlock had finished filing away a few things about babysitting into his mind palace, he decided to have a look around the flat. It would be interesting to see what the Watsons' life was like when he wasn't around.

He immediately came across a room at the top of the stairs that was locked. He found the key under a small angel statue sitting on a small table in the hall beside the door. Inside there was a filing cabinet in one corner, a potted plant in another corner, a desk against one wall with a wooden box beside it that had a door on the front, and a wardrobe on the opposite wall.

Sherlock walked over to examine the wooden box. The door on it was locked. He noticed a stool next to the wardrobe which he assumed was used to reach items on top of it. He was tall enough reach his hand up and feel the top of the wardrobe without it. He felt the cold metal of a key and took it down to see if it would fit into the lock on the wooden box. It was no surprise that it fit perfectly. The lock clicked open.

Sherlock sighed when he saw that there was yet another lock on a small file box inside. What was with all of this secrecy? This time, it was a padlock with a four letter code. This brought back memories of his case with Miss Adler, the woman. The woman. He started to become distracted with his memories, but quickly focused himself to solve the task at hand.

It was clearly stated on the file box: Property of Mary Watson. He needed to figure out the four letter code. What could it be? Sherlock decided to start off having a bit of fun. Maybe Mary was as sentimental as Miss Adler had been. He put in the combination J-O-H-N. He tried the lock, but it wouldn't move.

I guess she wouldn't choose something that obvious. This much protection must be for something important, Sherlock thought. He chuckled to himself. Could she really be self-centered enough to make it—

M-A-R-Y. No luck.

There was no time for games now. He wanted to solve this case that he had brought upon himself before either the baby or Molly woke up. He needed to think. What code would Mary choose that even John would not guess? What would be something important to her? Then it came to him. He remembered the day that he had discovered her secret.

A-G-R-A.

Her initials. The lock clicked open.

Inside was something that Sherlock almost wished he had never seen. There were pages and pages on Mary's past. Every job that she had been assigned as an assassin was right here in the file box. At first he assumed that they were just paper copies of whatever had been on the computer file she had given John, but then he saw the date on the most recent one. November 12, 2015. That was only a few weeks ago.

Sherlock was shocked by what was written on the page:

Date Assigned: November 12, 2015.

Name: Mary Watson Code Name: Moran.

Employer: James Moriarty

Mission: Assassinate Mycroft Holmes and Sherlock Holmes.

Sherlock heard Rosie starting to cry downstairs. He quickly put everything back the way he had found it.

"Sherlock?" Molly called.

"Yes. I'm coming," said Sherlock on his way back downstairs.

"Where were you?" she asked.

"Bathroom," Sherlock responded.

"It's your turn," Molly said with a smile as she handed over the baby.

Sherlock glared at her. "Fine."

A/N: Thanks for reading and Merry Christmas! I will be posting a new chapter every day until Series 4.