"You don't seriously think Sadie Holland is responsible," Lestrade said for the fifteenth time as he, McMannis, Dyer and Sherlock got out of the car. The streets in Mousehole were so narrow and winding that he'd been forced to park it half up on the kerb behind a row of shops, leaving them to walk the short distance up the hill toward Brett and Sadie's old home.

"It's a hypothesis," Sherlock said snippily, shoving his hands in his pockets. The day had fined up into scattered clouds and bursts of late-autumn sunshine, but even his coat and scarf couldn't contend against the freezing wind. "And one you never thought of, because you were so in love with the idea of Sadie being the damsel in distress. Donovan said you found damage to the Hollands' home that looked as if it were the marks of domestic violence. We need to follow that up."

"I s'pose if Brett was abusive, Sadie could have just upped and snapped one day," McMannis said. "And then panicked 'cause she never meant to kill anyone-"

"Or Brett mightn't have been abusive at all," Dyer interjected. "Sadie could be a raging psycho who killed him on purpose and buggered off because she didn't want to do time."

Sherlock cast him a grateful glance.

"That's an awful thing to say about a missing woman," McMannis said.

"And there is our problem," Sherlock said. "We're never going to get anywhere unless you think of her like anyone else involved: a suspect. This is the right house."

Before unceremoniously leaving, the Hollands had lived in a tiny two-up, two-down semi in Southview Terrace. A beautiful cottage, all grey stone and white painted window frames, with a row of peonies on the windowsill (clearly fake, Sherlock observed with a little snort of superiority. Their colour was too vivid, their petals too perfect; besides, they were distinctly out-of-season). An added sunroom sheltered the front door from the bracing winds off the ocean. In it, they could see a ginger cat stretched out luxuriously on a battered brown sofa. It jumped two feet in the air when Lestrade knocked at the door. A plump, blonde woman in her late twenties answered it, holding a crumpled dish towel in one hand.

"Oh." She switched the dish towel to her left hand and held her right out to shake Lestrade's. "Sorry, I completely forgot the time. Beth Tuckness."

Lestrade introduced himself, then McMannis, Sherlock and Dyer.

"It's terrible what happened to those poor people," she said, her pink-and-white face crumpled in concern. "Have they found the woman and her little girl yet?"

"Not yet," Lestrade said. "But we're working round the clock to find them. Anything you can show us from when you moved in would be really helpful, Mrs. Tuckness."

"Well, come through," she said, waving them all in and shutting the door behind Dyer. "Not you," she said to the cat. "Stupid boy, he knows he's not allowed in when I've got Callum on the floor."

Callum was, apparently, her baby son: a roly-poly, red-faced, spittle-cheeked specimen of seven months. He was on a play-rug in the middle of the living room floor, and the adults in the room had to practically step over him to get past. The cottage was beautiful, but miniscule, almost as if it were a scale model of a real house.

"Not much to see," Beth said apologetically. "But in here…"

She led them to the kitchen, just off the living room and giving onto the front windows where the peonies were displayed. She pointed to the stone floor. "That."

Sherlock got down on his knees to have a closer look at the crack running along one of the stone tiles, ending in chip the size of a man's thumbnail, displaying the lighter, rougher stone beneath. They were all silent for a minute or two while he looked.

"Dropped something," he finally said, getting to his feet. He looked very close to disappointed. "Nothing sinister, I wouldn't think. A crock-pot or a meat tenderizer, perhaps. The floor is sandstone. Sandstone is fragile. Show me the damage to the walls."

The damage to the walls, Beth explained in faintly embarrassed tones, was in the bedroom upstairs. She led them up a winding staircase set at a breakneck steepness until they were all in her bedroom, a small room painted a nauseating shade of apricot, and with a blue bedspread that was so jarring it was obvious that whoever had painted the walls, she hadn't. Directly opposite her bed was an ensuite doorway. It was obvious that it had all been one larger room at one time, and the ensuite had been created later. The wall between them was little more than flimsy gyprock. In it was a hole, deep and almost clearly-defined at its borders. Sherlock examined it, running his fingertips along its edge. Then he turned to Beth.

"We're about to discuss a sensitive kidnapping case," he said. "Get out."

"I beg your pardon?"

"Thanks so much for letting us do this, Mrs. Tuckness," Lestrade said, instantly his most charming. "I'm wondering, could you please leave us up here for a minute or two?"

With a miffed glance at Sherlock, Beth swiped at the hair pulled behind her ears and left. Sherlock waited until they could hear her footsteps on the floorboards below until he said, "Dyer, here. How tall are you?"

"Six foot, half an inch," Dyer said.

"Excellent. Stand there." Sherlock positioned Dyer so roughly against the hole that is was more of a push than a nudge. Then he beckoned to McMannis. "You, McMannis?" he asked. "How tall are you?"

"Five ten."

"Over here…" He pulled at McMannis's arm. "As you can see…" He pointed. "The hole is virtually head-height for Dyer, who is half an inch shorter than Brett Holland. Now if McMannis took a swing at him, he'd have to reach in a most unnatural way to reach his head, which seems unlikely."

"So what if their positions were reversed?" Lestrade asked. "Dyer trying to punch McMannis?"

Sherlock repositioned the two men. He pulled Dyer's arm out as if he were a puppet and demonstrated. "A straight swing lands too high," he said. "He'd have missed by quite a lot."

"Okay, so what happened, then?"

"Someone punched the wall," Sherlock said. "The shape of the hole is quite distinctive. But Brett wasn't punching Sadie, and Sadie most definitely wasn't punching Brett."

John was home, packed, and ready at the promised time; Molly took the keys to fetch the car while he gathered up the last of Charlie's things. As she idled on the kerb and he hauled things into the back seat, he realised that he'd packed as much for Charlie has he had for himself and Molly put together. Charlie, predictably, started whining as he was clipping her into her car seat. She was not a fan of the car at the best of times, and had never been on a five-hour drive. With four planned stops, the trip was probably going to take seven hours.

Molly turned on talkback radio before they were out of Central London, since it seemed to have a better effect on Charlie than music. John drew out his pencil and the scrap of paper he'd written Trish's contact details on and started calculating various dates. Mostly in February and October, Molly saw, surreptitiously glancing at the paper while they were stopped at a roundabout waiting for right of way.

"What are you doing?" she asked.

"Nothing," he mumbled. Then, remembering a promise not to dismiss his wife with 'nothing', he said, "Molly, if you were two weeks pregnant, would you know?"

"I… er, what?"

"I spoke to Sadie's neurologist this morning," he said, trying not to mumble his words into his collar. Details of exactly how he knew Sadie's neurologist could wait. "She said she last prescribed Epilim for Sadie in February of 2014. Even assuming Maisie was born at 37 weeks, she still would have been two weeks pregnant the day she got that prescription. I'm trying to figure out if she knew."

"Well, that's the wrong question," Molly said promptly.

"… Is it? Why?"

Molly paused to check her shoulder and changed lanes. "Remember," she said, "when we arrived in Rome on our honeymoon, and I had that shocking headache?"

"Yeah," John said, even though of all the memories he had of his honeymoon, Molly's headache was not one of them.

"I would have been about two weeks pregnant with Charlie," she said. "And we were already wondering… we were talking about it on the train, remember?"

That he did remember.

"And I said I didn't want to spoil things by testing so early because who knew what kind of test we'd buy in a foreign country where neither of us spoke Italian, and the cheaper tests throw false positives all the time. So if the question is 'did Sadie know she was pregnant', well, she could have. But anyway, so I had a bad headache, I felt terrible. You gave me a couple of paracetamol tablets on the train—and John, I never told you this before, but I said 'thank you', took them to the dining car to get a bottle of water, and I… on the way back, I dropped them in the ladies' toilet."

"Why?"

"Because all I could think was, I didn't want them to hurt my baby."

John looked indignant. "I would never-"

"Exactly, John! We're both doctors, you knew I could be pregnant, you gave me something both of us knew would be completely harmless if I was—and I still couldn't bring myself to take them, just in case! And Charlie was… not really planned… well, we weren't intending to have her so soon. If I'd been trying to have a baby for fifteen years, and somebody changed my medication, I'd have been asking about it, Googling it, finding out everything I could about whether it was going to hurt my baby if I was pregnant."

"And Sadie didn't do that."

"If she had, she would have found out how bad Epilim could be."

"So what are you saying?"

"She wasn't pregnant," Molly said. "And she knew she wasn't. She wasn't trying."

"Wasn't trying? She was doing IVF."

"Yes, but was she for all that time? I mean, I…" Molly blushed and suddenly pursed her lips. "Er. Never mind."

John smiled. "You've got to tell me now, Lolly," he said. "Come on, it might be important to help Sadie and Maisie."

Molly, still as red as fire, took a breath and fortified herself. "All right," she said. "When I turned thirty, which is before I met you…"

"I know."

"I actually looked into IVF. I wanted a baby and I… didn't have a boyfriend… well, I looked into it. And, um."

"Um?"

"Nobody does IVF for fifteen years, John. It's expensive, it's upsetting… I spoke to women who said it took over their lives and wrecked their marriages. And if it hadn't worked for a few years, you'd get reproductive counselling pretty much telling you to give up for the sake of your sanity."

Both of them heard a tinny jingle as Charlie dropped her toy elephant onto the floor of the car. Automatically, John reached into the back seat to retrieve it before she could fuss.

"Say please?" he coaxed.

Molly smiled. "She's too little, John."

"That's what she wants you to think." John surrendered the elephant, and Charlie grabbed at it and then gleefully shoved it in her mouth. "Okay," he said. "I don't understand."

"They gave up, John. Sadie and Brett. If you look up when exactly they went through those IVF rounds, I think you'll find they weren't having any close to the time Maisie was conceived."

"So Maisie was just, what, a… miracle?"

"Maybe," Molly said thoughtfully. "Maybe she's adopted."

"Why do we have to pretend we're fucking?" Donovan complained.

She was sitting in the passenger seat of Lestrade's car, which was parked outside of the local fish and chip shop on the promenade. It was now after dark, and they had been sitting there in the shadows for a minute or two, with Lestrade in the back seat, discussing Sherlock's plan to gather local information on the Hollands.

At her choice of words, Sherlock visibly flinched. "I told you," he said long-sufferingly. "Our having some sort of clandestine love affair is the only believable reason we would be in a tiny village in Cornwall, in November, where there also happens to be a murder investigation going on."

"You know I'm going to have to out myself as a detective eventually."

"Yes, that won't matter, as long as we get the information we need beforehand."

Donovan pursed her lips. On the one hand, she'd sooner eat broken glass than pretend to be Sherlock Holmes's girlfriend. On the other… there was a sick woman and a toddler missing, and they might still be alive. And Sherlock's plan might have been unnecessarily complicated—but it might also just work. A lot of the crazy things he did worked. It was why Lestrade let him do them. "Why don't you go in there with Lestrade and pretend to be his boyfriend?" she demanded, turning to point a finger at him.

"Oy," Lestrade said. "Do you want to go back to working traffic for the rest of your life?"

"Look." Sherlock sighed. "It really is incredibly simple. We go in there, order some fish and chips, and act vaguely affectionate toward one another. Enough to satisfy bystanders that we're having an affair and we don't want your husband to find out, so we've chosen this out-of-the-way spot as a love nest. It's plausible enough. The place must have had other couples here for the same reason; it's picturesque and right on the water."

Donovan narrowed her eyes. "What, exactly, is 'vaguely affectionate'?" she demanded.

"And for God's sake, don't call me Sherlock," he said, as if he hadn't heard her question. "So far as I'm aware, I'm the only Sherlock in the world. I don't want anyone wondering about the name and Googling it, which will instantly bring up a picture of me and identify me as a private detective. I'll casually ask some questions; you play along. We get our food. We leave. Does that sound difficult to you?"

"No."

"Then let's go."

"No." Donovan crossed her arms. "You can go in and pretend to be whoever you like, Sherlock, I'm—hey!"

Sherlock had got out of the car, crossed to the passenger side, opened it, and was dragging her out by one hand. He pulled; she pulled back. Being slightly stronger, he'd just managed to drag her out of the car and under the light of a street-lamp when a pair of men on the other side of the street saw them.

"Oi," one called, stopping. "Everything all right over there?"

"Yes, sorry, we're only playing," Donovan called back, smiling. Then, through her teeth, "I hate you, Sherlock."

"Good for you," he said, leading her by the hand more easily now. The two men were still watching them. "Play along, and you can hate me all you like."

The shop door jangled as Sherlock opened it and stepped intrepidly in, still with Donovan by the hand. There were no other customers, as well they might expect at a cold and dreary time of year. It was a surprise the place bothered to trade. A single worker stood on duty; a stout, middle-aged woman with a greasy face and stubby, wrinkled hands. Lamentably, she did not seem to be the chatty type.

Both Sherlock and Donovan looked up at the menu board for long enough for their faces to be clearly seen on the CCTV camera bolted to the wall, then Sherlock stepped forward and ordered—in an accent more like Lestrade's than his own. Donovan blinked, then remembered in time that she was being watched. As Sherlock stepped back, she linked her arm in his and—inwardly seething, outwardly smiling—nuzzled closer to him, taking great pains that her wedding ring could be seen. The woman behind the counter, having nothing much to do while the fryer did the work for her, started wiping down already-pristine benchtops.

"I saw two police cars parked on the promenade, lights going and everything," Sherlock said at length. "Someone get killed?" The question came out as a nervous joke.

"Oh, you didn't hear?" The woman pushed back one greasy strand of greying hair. "Murder, yes. A couple out on a boat, and their little one. They found the man in a suitcase, all chopped to bits."

Sherlock whistled. "Well, we've only just got in now," he explained. "Hadn't heard. That's horrible. Did you know them?"

The woman shrugged, turning back to the deep fryer to check on the contents. "They came in to the shop every now and again," she said. "She did, anyway, pushing the little one in her pram. Poor souls."

"He wasn't a fan, then?" Donovan said lightly. "Health freak?"

She snorted. "Drank, or so I heard," she said. "Mrs. Cardy—she lived next door to 'em—she said they got up some fearful rows. The whole street could hear."

"She have bruises, then, when she came in?" Donovan persisted.

Sherlock shot her a warning glance, then reached out, brushed a tendril of hair off her forehead and kissed her nose. After a second's pause, she laughed and drew closer, shoving her hand in his trouser pocket.

"Not that I ever saw," the woman said with a shrug. "Hey, you two, this is a shop, not a kissing booth. On your honeymoon, then?"

"Um… yes," Sherlock mumbled. Perfect: it sounded like a lie, because it was one, though not in the way the nosy locals might think.

"From London?"

Donovan nodded, suddenly looking sheepish. She drew Sherlock's left hand up over her hip, where it was highly visible.

"But we don't have definite plans," Sherlock said. "Can you recommend anywhere to stay?"

"The Ship Inn," she said immediately. "Further up on the promenade. My cousin Judith runs it. Tell her Rosen recommended it; she may be able to do you a good deal at this time of year." She turned back to the fryer to retrieve the oily mass of fish and chips, salt it and wrap it in paper. Popping it into a plastic carry-bag, she said, "All right, that's a large chips and two bits of battered cod - £11.35, thanks."

Donovan drew her hand out of Sherlock's pocket to pat her own. "Shit," she muttered. Then, in a whine, "Billy, I've only gone and left my purse in the car…"

With a wry smile at Rosen, Sherlock drew his own wallet out of his coat pocket. "I swear, my wife thinks I'm made of money!" he said, handing a couple of notes over.

"Very likely she does," Rosen said, narrowing her eyes slightly. "But Blind Freddy could see this one's not your wife."

Sherlock blinked. "I'm sorry?"

"Married couples don't carry on like you two do. Now, look, I'm open-minded," Rosen went on, holding her hands up. "But not everybody is. Tone it down a bit, is all I'm saying. Tell Judith you're on your honeymoon." She held out a handful of change, and after a pause, Sherlock took it, putting it in his wallet and mumbling thanks.

"You two have a good night," Rosen called after them as Sherlock pulled the door closed after them.

"What the hell did you call me?" he demanded as soon as he'd shut the car door behind them. Uninterested in the food they'd bought, he passed it over the back seat to Lestrade.

"Billy."

"Oh, I'm sorry. When I said 'don't call me Sherlock', I should have specified 'don't call me anything stupid'." Sherlock stopped. "Wait. Who's Billy?"

"My little bruv, actually," Donovan said. "And you're just lucky I didn't call you the name on the tip of my tongue, which was 'Rahul'. Anyway, what the hell were you doing, kissing me on the nose?"

"I thought it a much more hygienic token of affection than kissing you on the mouth. Should I have done that instead?"

"Only if you wanted to lose teeth… shut up, boss, or I'll go tell Chambers you've both been sexually harassing me."

In the back seat, Lestrade was laughing so hard he was crying.

"Yes, shut up," Sherlock said to him without malice. "Well, Sally…" He turned the key in the ignition and reached over to turn the headlights on. "Time for you and I to check in at the Ship Inn."

"… What?!"

"You heard our helpful guide. She's going to tell everyone she knows that there's a shameless couple in town having an affair, and she recommended us to the Ship Inn. If we don't check in, she'll know something is wrong. We already dodged a bullet: she recalled that a suitcase had been found, but not that you'd been there as a detective."

"I'm not sharing a bloody hotel room with you!"

"Oh, will you relax? I'll sleep on the floor."