"Well, this all started when your daughter noticed something in the floor tiles. Trust the Goddaughter of Sherlock Holmes to pick up on something like that," Molly said, shaking her head at John. "She surmised that it may be some sort of a code, and Eurus confirmed that, deciphering it easily. This led us to digging out a brick near the fireplace chamber, where we found these," Molly said, pointing towards the brittle birth certificates.

"From there," Rosie continued, "Auntie Eurus determined that the code on the floor, which was a simple set of directions, led to something bigger than simply some old papers hidden in the wall."

Greg, still a little alarmed at the use of demolition tools in his beloved little house, looked like he wanted to start interrogating his wife and niece, but wisely refrained from doing so. Sometimes, he knew, you got better answers by listening rather than by asking.

Besides, Eurus was the most clever person he'd ever met, and Rosie was a brilliant young copper who had been raised by Holmes and Watson, and mentored by himself as well. She'd already learned every trick from all three of them and he knew he wouldn't get far at all with any of that ridiculous folly.

"The gaping hole where our bed normally sits held a rather macabre secret, darling," Eurus said. "Under the flooring we found the burial spot of four very young children. Rosie called it in and Charlie came out with a recovery team last night to collect them and take them in to the mortuary at St. Bart's." She watched quietly as some of the colour drained from Greg's face.

"Now, based upon my findings on my lunch break, what Mum found out in her examinations," Rosie said, nodding in Molly's direction, "and Auntie Eurus's deductions, we think we know what happened."

"The remains were clearly not recent, and I determined with reasonable probability that they were the same children as had been listed on the birth certificates we found in the wall. They all bore the same family name," Molly said. Sherlock shuffled in his chair, then settled, his focus on his wife undivided. "The baptismal records indicate that they were siblings. Tragically, this area was one of countless areas worldwide that was stricken by the Spanish influenza pandemic in 1918 through 1919. It would appear that the children fell victim and passed away."

"Sadly, according to burial records from the local cemetery," Eurus continued, "it would appear that the parents also succumbed to the influenza. Based upon the ages of the children and the records from the church, the parents passed away several months before the babies did."

"Now, according to the baptismal records, there was one older sibling, I believe she would have been around 14 years old at the time of her parents' deaths," Rosie continued. "She was likely left to care for her infant siblings, and I would imagine she was rather overwhelmed by the whole thing, especially when they fell ill with influenza. We have no proof of course, some of this is based upon science and observation, conclusions drawn from evidence… and some of it is educated guesswork based upon the facts we've been able to determine."

"She was just a young girl herself," Molly said, "and I believe that the stress from the circumstances may have broken her. When her siblings passed away, she may no longer have been emotionally equipped to deal with it. As best as we can determine, she buried her poor baby siblings in the floor. From there, she likely detached herself psychologically, which is where the code and the birth certificates came in."

"I think she was a very very clever girl," Eurus said quietly. "And as a result, she was very fragile. She may not have realized what she was doing when she buried them, but something in her own alternate reality wanted to create a way for them to be found and properly buried. She just didn't know how to do that. So she created what in her own mind was a simple code with a simple set of directions and used some sort of stain to apply it to the tiles. At the start of her code trail lay the birth certificates behind the brick she dug out. At the end of it, lay their burial spot. The poor girl must have spent a great deal of time in isolation to replace the flooring the way she did. It was meticulously done. Once she had enclosed the spot, which she must have done rather quickly, she would have had plenty of time to install the tiles."

"My God," John muttered. "Do you know what happened to her?"

"Unfortunately dad," Rosie said, "the records we were able to find show no indication of her fate. We can only hope that she found someone to care for her. Our conclusions end here."

"I've managed to arrange for the children to be buried in the cemetery in their family plot, with their parents," Molly said.

"So darling," Eurus said, turning to Greg, "I hope you understand why we had to dig up the bedroom floor and pry a brick out of your fireplace." She smiled hopefully. Greg's frown of curiosity had turned into one of sadness at the tale the ladies had uncovered.

"Of course," he said, reaching up with his hand to brush the curls aside and leaning in to kiss her forehead. "Considering what's been under the bed all this time, I'm glad you found them sooner rather than later. I don't know when I was going to get around to it, but I was going to take the flooring out anyway, no worries, love."

"So, this is quite the mystery you ladies have come across," Sherlock said, sounding mildly impressed – which for Sherlock, meant that he was in actual fact, VERY impressed.

"You're not surprised they solved it all on their little owns?" John chided jokingly, giving him a gentle jab in the shoulder. John knew Sherlock was bloody proud of the ladies, just as he was himself, but he just couldn't help teasing him a little bit.

"Don't be ridiculous, John," Sherlock laughed, understanding the context of John's harassment. "I mean really, look at them," he said, waving his hand proudly towards their Baker Street Girls. "A doctor, a copper, and a clever woman with a knack for puzzles. Of course they were going to solve it without us. How could there be any doubt?"