In more than fifteen years of living in London, this little enclave within the city was somewhere Molly had never ventured. With its neat, flat-fronted Victorian terraces, each seemingly with a different shade of brightly-painted front door, it was like London from a bygone age, and hard to believe they were less than a mile from the South Bank.

Sherlock had paused before ringing the doorbell, apparently examining the doorframe, running a gloved finger along the paintwork and around the Yale lock. And now, they were being led through the narrow hallway to the back sitting room of one such house by Sherlock's client. The room, like the other areas of the house that Molly had glimpsed, was clean and neatly-arranged, if slightly dated in its decor (not that Molly felt she could judge, given that she lived with that horrible avocado bathroom suite for four years before it finally got too depressing for her). It took her a few moments to notice the tabby cat that had been curled up in a wicker chair, and the cat apparently noticed her around the same time, because it dropped lazily down onto the floor and padded towards her, immediately curling itself around her ankles.

"Oh, I'm sorry, that's just Oscar - you're not allergic, are you?" the woman asked, moving to retrieve the cat.

"No, it's fine," Molly replied, stooping to pet the cat's head. "I've got one of my own."

Several times over the years she'd thought about getting a companion for Toby, but had worried (in opposition to her feminist sensibilities) about the number of cats a single woman had to have before she officially turned into a Cat Lady. But now that the being single thing was no longer applicable, maybe the time had come to actually do it. And Toby could do with an ally against Tom's dog, Rufus. Sorry, their dog, Rufus.

A good few moments must have passed while Molly indulged Oscar, and when she eventually straightened up, Sherlock was watching her with an expression of mild exasperation. Making friends with his clients' pets clearly wasn't part of his usual methods of working. She stifled a smile.

"Mrs Armitage, you recently suffered a theft, I understand?" he said, his hands grasped tightly behind his back as he took a couple of strides towards the bookshelves over by the chimney breast.

"Yes, that's right," the woman replied, a little hesitantly. "Please do sit down, by the way,"

She hurried across the room to clear a stack of files, exercise books and an assortment of pens from the small sofa. Molly could see that Sherlock was reluctant, but she could also tell that he was trying to maintain his client's trust, and so he acquiesced. A second after they both took a seat on the sofa, it became apparent that some of the springs were long gone; the sofa seemed almost to fold in on itself, and Molly had to grab the arm to prevent herself from ending up in Sherlock's lap. Just to add to the day's other indignities.

"Can I get you both a tea?" Mrs Armitage asked.

The suggestion suddenly made Molly realise just how much she could do with a cuppa, but before she could say anything - and probably deliberately so, she suspected - Sherlock had declined the offer. Their host pulled the chair out from under the desk in the corner, and took a seat opposite them, folding her hands in her lap. She smiled nervously, her eyes flitting between the two of them.

"You're a teacher, Mrs Armitage?" Sherlock asked - or rather he used the intonation of the question, but Molly knew he rarely asked questions to which he didn't at least strongly suspect the answer.

"Oh - yes," she replied, a little surprised. She gestured to the jumble of things she had removed from the sofa and stacked on the desk. "You caught me in the middle of my marking, I'm afraid. My Year Threes are learning about the solar system."

Molly stifled another smile, feeling Sherlock stiffen slightly beside her.

"And you weren't at home when the robbery occurred?" he continued, unabated.

"I got back from the shops and the house was just...ransacked," she replied, with a sigh. "This room in particular. Drawers emptied out, books all thrown on the floor, just...mess, everywhere."

Sherlock gave what sounded like a sympathetic hum, bridging his fingers underneath his chin.

"And you found that this item, this...doll was missing," he continued.

She nodded.

"Yes. It was stupid, really. I never should have kept it somewhere it was so easy to find," the woman said.

"Was anything else stolen in the break-in?"

"Not really; nothing worth anything," she said. "A few necklaces and brooches, but they were just costume jewellery, really."

At this point, Mrs Armitage's cat sprung up onto the arm of the sofa closest to Sherlock, and immediately proceeded to stroll onto his lap. Molly looked from the cat to Sherlock, watching Sherlock's expression as he battled the urge to forcibly propel the beloved Oscar from his knee, and instead maintain a rapport with the client. Undeterred by the lack of obvious welcome, the cat was now stalking around Sherlock's lap in circles, claws snagging on suit trousers that Molly suspected cost more than her monthly mortgage bill. With the cat's tail now swaying inches from Sherlock's nose, Molly decided it might be time to take pity on them both; she gently reached over and scooped the animal into her own lap, where it immediately settled down and offered itself up for stroking.

"My husband wasn't keen on them either," Mrs Armitage said, with an expression of recognition.

It took a moment for Molly to realise what was being inferred. She shot a glance towards Sherlock, and then back to the client again.

"Oh, um, no, that's-" she began. "Earlier, I meant, I have a cat. We-"

She wasn't sure where she was going to go with the rest of that sentence, but thankfully, just as her cheeks were threatening to flash-fry the rest of her face, Sherlock interjected. Although Molly was fairly sure it was less about saving her blushes, and more about his dislike of superfluous chat.

"Out of interest, Mrs Armitage, how did you come to own this particular doll?" he asked. "I assume you must be a collector?"

The same thought had crossed Molly's mind, but nothing about Laurel Armitage or her house seemed to support that idea.

"No," she replied. "It's funny; I bought it from a second-hand toy shop years ago, for my niece, but by the time I actually saw her to give it to her, she'd grown out of playing with dolls."

"But you knew it was worth a lot of money?" Sherlock prompted.

"Not then I didn't. That happened later - quite by accident. I didn't tell anyone, and I didn't know quite what to do with it, to be honest - it all seemed a bit ridiculous. I suppose in the end I thought I would hang onto it, in case the money came in handy at some point." She gave a rueful smile. "In hindsight, I probably should have just sold it there and then."

"In your email, you mentioned a ransom note," Sherlock continued. "Left behind by the thieves. May we see it?"

Mrs Armitage got to her feet, pausing a moment, brow furrowed, as though trying to remember something. While she was searching through her desk drawer, Molly looked across at Sherlock, who in turn aimed a dark stare at Oscar the cat, while attempting to brush cat dander and stray hairs from his trousers (she definitely would not be mentioning the tiny hole in the fabric that she had spotted, given how close it was to his crotch).

The client returned, unfolding a piece of white paper before handing it to Sherlock. He glanced at it for a second before passing it across to Molly; it was a regular piece of printer paper, bearing a typed note:

Your property is in a safe place

£10,000 gets you a location and a locker key

You'll be hearing from us

"And have you?" Sherlock asked. "Heard from them?"

Mrs Armitage shook her head.

"Even if I did, I don't have ten thousand pounds to give away."

"Have you told the police?" Molly asked, setting the ransom note aside. The more time that passed, the more she was starting to see what drove Sherlock to take this case; there was definitely something shady going on.

"I didn't think it was worth it," the woman shrugged. "The police don't tend to bother very much with burglaries these days."

"Even for such an expensive item?" Molly replied. It suddenly occurred to her that she might be trampling all over some careful line of questioning that Sherlock had planned, but when she glanced across at him, he just seemed to be awaiting his client's answer.

"They would probably just ask me why I hadn't had the doll properly insured," she sighed. "Which is why, Mr Holmes, I came to you instead."

Sherlock nodded thoughtfully, leaning back on the sofa and crossing one leg over the other.

"It's quite a story, Mrs Armitage," he said. "In fact, your story has inspired me to come up with one of my own, and you'll have to forgive me, but I think might be even better. Would you like to hear it? Of course you would - it really is pretty good. Mrs Armitage, your home was not burgled - the original lock mechanism is still in place, and there is no evidence whatsoever of damage to your front door, except where your dim-witted pet has clawed at the paintwork to gain entry, despite a perfectly good cat-flap at the back of the property."

Despite the direction in which this story was heading, Molly couldn't help but feel a protective pang towards the cat still curled up on her lap. For his part, Oscar had turned his head to watch Sherlock as he got to his feet (leaving Molly to recover her balance on the blancmange-like settee).

"Also, you said that the robbers threw your books on the floor, but these books," Sherlock continued, sliding a volume from the shelf - "are still covered with a noticeable layer of dust; much more than a fortnight's worth. Three months ago, you sold your car, and until a few weeks ago, you were advertising for a lodger - not the actions, I would surmise, of a person who was sitting on a highly valuable collectable."

Molly watched the woman's posture instantly change, her fingers digging into the cushion on her chair.

"I...I sold my car because I barely used it," she said, blinking rapidly. "And yes, I was thinking about letting out my spare room, because I've never got used to living by myself. That doesn't mean-"

"Mrs Armitage, only five of these dolls were ever produced," Sherlock continued. "One of them is owned by a New York socialite, another is in the collection of a Saudi prince. And a month ago, one of them was sold at a London auction house to this man…"

He held up the screen of his phone, displaying the photograph Molly had seen earlier; the well-groomed man in his forties.

"Russell Marriott," Sherlock said. "Who, for reasons best known to himself, has spent almost half a million pounds on Barbie dolls over the past fifteen years. He contacted me two weeks ago, frustrated with the police's response to a break-in at his Knightsbridge home, where - amongst various other tasteless knickknacks - he was robbed of his latest acquisition. So, the question is, Mrs Armitage, how did you orchestrate such an audacious burglary? And where are you keeping the rest of your spoils?"

The drama was too much for the cat, who shot off Molly's knee and into the kitchen.

"What?!" Mrs Armitage responded. All the colour had drained from her face, and Molly was momentarily worried that she might have to administer medical treatment to an actual, live person. "That wasn't...I didn't...how could I have?" Shen floundered around for a moment, and Molly almost felt sorry for her; after all, she knew what it was like to be on the receiving end of a very intense Sherlock Holmes stare.

"What I'm telling you is true," the woman said, once she had recovered herself. "The note I gave you, the locker..."

"Oh, the bit about the locker is true," Sherlock said, adopting a more airy tone. "Well, partly. But only because you put the doll there."

Molly heard an audible gasp from the client.

"Your disguise was rubbish - you were clearly recognisable on CCTV," Sherlock added. "There's very little that's more disappointing in my line of work than a mediocre disguise. And if you will use your own computer to type a ransom note, at least make sure your printer doesn't have a very conspicuous and incriminating fault." He held up the piece of paper Mrs Armitage had earlier produced. "This has the exact same smudge pattern as the papers you cleared away from your sofa when we arrived."

He crossed back to the sofa, hoisted up Molly's bag from its resting place by the foot of the sofa, and like a very posh and well-dressed magician, produced the Barbie doll from its midst.

Mrs Armitage was visibly shaking now, as she raised her hand to cover her mouth.

"You're a primary school teacher," Sherlock continued. "I'm going to assume that you recognise this child?"

Again, he held out his phone, this time displaying the photograph of the young girl, her hair in plaits, beaming through a gap in her teeth.

The client nodded, swallowing hard.

"She's...she's one of my pupils. Emily."

"Emily Flynn," Sherlock said. "You may not have carried out a heist on a mansion in Knightsbridge, but you did steal from a six-year old girl in your care...didn't you, Mrs Armitage?"

He had lowered himself back onto the sofa again by now, once again adopting a more measured tone with his client (just as well, Molly thought, as very soon she'd probably need a paper bag to breathe into). After a few long moments, where Mrs Armitage looked as though she was in agony, she finally took a deep, shaky breath and took up the story.

"It was a Saturday, a few weeks back," she said. "The school was having a fayre, the PTA raising money for something-or-other, and it's more or less expected that staff will give up their weekends to help out, on top of everything else. I was stuck on the second-hand toy stall, and Emily was there with her family, and she picked out the Barbie. I didn't think anything of it until she came into school on the Monday with the doll, and gave it to me to put in her drawer until break time. I knew immediately that the diamonds were real - my father had a little jewellery shop in Holburn when I was growing up. So...I took it."

Sherlock flicked through his phone and Molly saw him pull up an email.

He cleared his throat, stagily, and read, in a slightly sing-song voice.

"'Dear Mr Sherlock. Please can you help me find my doll? I lost it at school. I saw my teacher put it in my drawer, but then it was gone. Mummy says I'm not allowed to take toys to school anymore, because I don't look after my things. Maybe there is a black hole in my classroom?'"

Molly watched as Mrs Armitage dropped her face to her hands.

"Black hole," Sherlock added. "Well, at least she's learning about the solar system, so I suppose you should be commended for that, at least."

Molly shot Sherlock a look; aside from the fact that the nearest black hole was something like three thousand light years away, his comments weren't helping.

"You needed the money?" Molly said. "And you saw the opportunity?"

Mrs Armitage sighed.

"Do you know how difficult it is to live in London on a teacher's salary?" she said. "And I have debts, debts I've never told anyone about. And I thought...it was a doll - a six year old child wouldn't care if it cost six pounds or sixty thousand pounds. She probably has a box of dolls just like it at home - except they wouldn't pay off my second mortgage."

"So why all this?" Molly asked, slowly. "Why pretend you'd been robbed? Why ask for help?"

"Because she knew about the theft from Russell Marriott's home, Molly," Sherlock said, in a gentle tone. "Didn't you, Mrs Armitage?"

She nodded again, hugging her arms around her middle.

"It was reported in the press," Sherlock said. "Now that you no longer drive to work, you read the free newspaper on the bus, and you saw an article about the burglary. And then you panicked - not only because it confirmed you were in possession of stolen goods - twice stolen goods, technically - but also because even if you were able to somehow return the doll, you would be back to square one. So you needed to come up with a way to both keep the doll and paint yourself as an innocent victim...and of course, you couldn't know that Russell Marriott would also become a client of mine."

It was apparent from the woman's reactions that Sherlock's deductions were more or less spot-on; she now wore a resigned, anguished look - and more than that, Molly could tell, she was mortified, too.

"But you have to believe that I have absolutely no idea how the doll ended up on the toy stall, Mr Holmes, I really don't," she pleaded.

"I do believe you," Sherlock told her, matter-of-factly, getting to his feet again. "And on the basis of this little experiment, I wouldn't advise a change in career."

Realising that Sherlock considered his work here done, Molly scooped up her bag and followed him towards the door.

"Are...are you going to tell the school?" Mrs Armitage suddenly blurted. "I could lose my job, and then I would lose my pension and I-"

"It wouldn't be worth the paperwork for the police," Sherlock replied. "And besides, Russell Marriott simply wants his property back, and probably won't ask questions."

With that, Sherlock swept out into the hallway, leaving Molly faltering for an awkward moment, left alone with Sherlock's client. She sometimes wondered at what point in his life Sherlock had decided to dispense with the beginnings and endings of conversations.

She and Mrs Armitage looked at each other for a few seconds, each seemingly unsure what to expect from the other.

"Okay, so…" Molly said, shifting from one foot to the other. "I...um, I really like your cat. Thank you."

Molly briefly clocked the baffled expression on the woman's face before she quickly turned and followed Sherlock out of the house; if she was cringing any harder, she would probably rupture a nerve in her face. And she wasn't sure what possessed her to thank the client - it was probably just a politeness reflex that she'd never been able to shake (not something that troubled Sherlock, obviously; Molly was starting to see why John had such a short fuse around him).

Sherlock was waiting outside on the pavement for her, turning his head when she approached.

"Molly, you didn't just compliment our client on her cat, did you?"

"No," Molly replied quickly, not meeting his eye.

She heard him utter a short, sceptical "hm". Although it occurred to her, with a smile, that Sherlock had described Mrs Armitage as their client. As they started to walk, Molly shrugged her bag off her shoulder and held it out to Sherlock, who looked quizzically at her at first, before realisation dawned and he instead rolled his eyes.

"Oh, we're really doing this, are we?" he sighed.

Gone were the days, though, when she would be cowed by his condescension. Instead, Molly continued to nudge the bag at his hip until he had no choice but to take it.

"I guessed right," she smiled. "And we had a deal, Sherlock."

Raising his eyes to the heavens momentarily, Sherlock grasped her duffel bag in his fist as though he was carrying a particularly malodorous rubbish bag out to the bin (something Molly was fairly sure he'd never done, unless on the orders of Mrs Hudson).

"As it's a pleasant day, I was going to suggest we walk part of the way," he said. "But in light of this" - he brandished the bag, with a faint expression of disgust on his face - "I think another taxi might be in order."

Molly smiled; there was a possibility she might put him out of his misery a little later on, but for now, she was enjoying this far too much.