Molly's kisses are nothing like John's, which should come as no surprise. They taste faintly of artificially sweet cherries. When Sherlock pulls away and licks his own lips, the flavour of the pink gloss clings to them. Taste aside, the contours of her mouth are wrong. Molly's lips are thin, where John's had a pleasing fullness. And as for the technique …

He feels an acute pang of regret as he remembers how John kissed him. It was an odd but intriguing combination of tender assertiveness coupled with a soupçon of bemusement that evaporated just before their lips brushed. It was if John couldn't quite believe where their close and complicated friendship had taken them, but he was glad of it all the same.

Molly's kisses, on the other hand, started out tentative and progressed rapidly to audacious. Perhaps she is embracing the role she is meant to be playing, but he doesn't think so. They both know if it wasn't for the investigation they were currently engaged in, he wouldn't be holding her in his arms. She is taking advantage, he decides. Indulging herself in what he'd thought was a long discarded fantasy.

With a suppressed sigh, Sherlock realises the situation is probably difficult for them both, but in completely contrary ways. She wants what she can't have and he has what he doesn't desire. It goes without saying that life isn't fair, but sometimes the degree with which the lesson is reinforced beggars belief.

It wasn't his idea to spend his Saturday night on a dance floor swaying to sultry rhythms, lights pulsing down on him in lurid bursts of colour whilst the other dancers undulated against one another in a graphic exhibition of ritualised foreplay. Nor was it his idea to engage in a public display of lovemaking. No, those bright notions had been down to Lestrade because in his view, re-enactments had to be comprehensive in every detail if they were to be effective. They are engaged in the uncomfortable spectacle because these were the last known actions of one Emily Case – the sister of Lestrade's neighbour over the road – who had gone missing, but not in a way that would officially interest the police. They are hoping to jog the collective memories of the club's patrons, prising from them two week old recollections of the woman and of the man with which she was last seen dancing when her flatmate had left the club to make her own love connection.

Molly teeters on stiletto-spiked heels as she steps out of Sherlock's embrace. She pivots sharply, arms raised above her head, and then leans inward, resting her back against his chest and cupping her hands at the nape of his neck as she sways her hips provocatively. Her perfume fills his nostrils. It's something new and unfamiliar to him; a complex blend of floral and herbal notes that's far more sensual than the cheap cologne and body spray the rest of the club's inhabitants have bathed in, clearly expensive, and thus obviously a treat she had purchased for herself.

Involuntarily, his body reacts, penis twitching against the denim of his jeans to signal that although his brain might be indifferent to the idea of sex with Molly or anybody else who wasn't John, it holds the minority opinion. His hands echo his penis's treacherous reaction, coming unbidden to rest splayed possessively against Molly's hip bones, anchoring her close as he follows her lead. When the beat changes, telegraphing the end of the song, Sherlock breathes a sigh of relief as Molly lowers her arms and self-consciously moves away.

The dance floor clears out as the DJ takes a break. Business becomes even more brisk at the bar. Lestrade and Donovan redouble their efforts, questioning customers as they flash photographs of Emily Case.

Molly remains on the dance floor bathed in the light of a baby spot; one more bit of theatre they'd arranged with the management. In the clinging, thigh skimming, electric blue dress and high heels that set off her legs to their best advantage, bold eye makeup far more daring then she might normally wear, and a wig of tumbling auburn curls, she looks quite unlike her normal work-a-day self. It would be easy to take her home, make love to her, and pretend she was nothing more than a one night stand, but Sherlock is doubtful that either his sense of honour or Molly herself would cooperate with the fantasy.

When he finally gets to the head of the queue he orders a large vodka tonic for Molly. He's tempted to order one for himself but because he is at work, he refrains and sticks to water. He leaves the questioning of the bar staff to Lestrade and Donovan but scans the crowd of patrons and evaluates their reactions out of habit. It is clear that at least some of them remember Emily Case. There is recognition in their faces as they stare openly at Molly and scrutinise him with narrowed eyes, trying to place where it was that they've seen him before.

The new pair of designer-knock off shoes make Molly's feet hurt. He can see it in her eyes and in the way she discreetly flexes her arches as she stands there on display. Sherlock offers his arm to lead her out of the glare of the spotlight. It's the least he can do. They sit at a small table, still in ready view of the patrons as the DJ resumes his place behind the mixer board. She takes a bird-like sip from the tall, perspiring glass he sets in front of her and then gives him a nervous look, her eyes flitting to meet his and then away again. "Was I all right?"

At the bar, a man is nodding in response to Donovan's query and she is looking keenly interested, jotting down his statement in a small notebook. Reluctantly, Sherlock has to concede that the re-enactment has been at least a partial success.

"Fine," he replies curtly. His thoughts aren't wholly on the business at hand. As he toys with his water glass the tightness in his groin gradually subsides, leaving a dull sensation of frustration in its wake. He wonders at Lestrade's motivations. Had the inspector offered the puzzle of Emily Case in good faith, seeking an unconventional solution to a problem that, strictly speaking, was out of his bailiwick? Or was his well-meaning guardian angel engaging in a bit of less than subtle matchmaking under the misguided belief that if they were thrown together often enough, Molly would offer the same steadying presence in his life that John had. Sherlock finds himself resenting Lestrade's intentions, even if they were strictly professional.

He glances up from his glass and watches Molly nervously sip at her drink. It's not just the re-enactment that fuels her jitters, he intuits. She's acutely aware of how awkward the atmosphere has grown between them; of the conflict between their bodies' desires and what would be best if they are to keep their friendship intact. She doesn't know about the true nature of his relationship with John. Or at least Sherlock doesn't think she does. With Molly, because she can be uncannily perceptive – at least where he's concerned – it's difficult to judge. But she does know her own mind, and Sherlock knows that she won't willingly step into somebody else's shadow.

"Molly," Sherlock begins, but then he trails off, not quite sure how to frame a conversation that he doesn't want to have.

It seems obvious that she doesn't want to go there either. She tips her glass back, throat working as she drains the last of the cocktail from around the ice. "Let's dance," she says, and extends her hand to him.

Though the song is slow they stand apart as they perform an awkward box step, no longer playing the characters of Emily Case and her mystery man. Instead they smile tightly as they almost, but not quite look at each other, like two strangers on a blind date who have sized one another up and found the other wanting. It's almost a relief when a heavy hand comes down on Sherlock's shoulder and pulls him bodily out of Molly's embrace.

A man. A tall dark-haired man, dressed in jeans and a white button down shirt, similar to the one Sherlock is wearing, stares down into Molly's face. He seems perplexed.

"Do I know you?" Molly asks. She is as confused as the man appears to be.

"It's me, Linda, — Gary."

Sherlock steps backwards, jostling the dancers behind him, and gives Molly a silent nod of encouragement. He is forced to lip read the resulting conversation, even though the pair stand within an arm's reach. The music changes, becoming deafening. It's heavy with a bass beat that reverberates through the floor.

"Gary … yes … of course." She stammers, clearly thrown by the situation, but gamely soldiering on. "I hadn't expected to see you tonight." She offers him a welcoming smile.

Gary reaches out and fingers Molly's wig, smiling back at her. "You've done your hair just the same." His smile drops. "But you've used too much stuff around your eyes. You have such nice eyes. You shouldn't hide them behind that muck."

Molly reaches up and touches a fingertip to her face, smearing the multi-hued eye shadow that had taken her an age to apply. "Sorry," she replies automatically. It's clear that she's decided that under the circumstances it would best to appear appeasing. Her posture becomes diffident.

"I didn't think I'd see you here," Gary says as if Molly hadn't spoken. Now he's looking at her with uncertainty. "You shouldn't be here."

"Why?" she asks. The dancers around them have a Brownian effect. Molly starts to sway to the rhythm of the beat, and after a moment, Gary does too.

He takes her hand, studying her face with a perplexed expression, as if he can't work out why Linda/Emily/Molly shouldn't be at the club. Until he works it out, he decides to take it in stride. He grasps Molly's hands, pulls her in for a twirl and rocks with her against his chest. Molly continues to play along, raising her arms as she had before and rolling her hips sinuously, imitating Emily once more.

Sherlock is bedevilled by a stab of annoyance as he views the display. Coldly, he dismisses the illogical reaction, rationalising it as impatience for matters to come to a head as once again Gary twirls Molly and then pulls her close to sway against his body.

Around them things are happening. Lestrade and Donovan watch events unfold with hawk-like fixation, as do a platoon of dinner jacket-clad bouncers. Even a few of the dancers, who were questioned earlier, bear witness with a sense of morbid fascination, knowing that something, although they can't be sure of what, is about to occur. The sense of breathlessness becomes choking and Sherlock forcibly reminds himself to exhale.

Gary freezes in mid step as something slips into place. He stares into Molly's wary eyes and his expression becomes haunted. "Because you're dead." Dispassionately, he grasps her by the throat and pushes meaty thumbs hard into her carotid arteries.

For a shocked second everything drops into slow motion as Molly's eyes go wide. Time snaps forward as she shakes off her surprise and reacts, driving her knee sharply into Gary's groin and then sending the long spiked heel of her shoe hard into his boot. She tumbles off balance, choking and coughing as, roaring in outrage, Gary lets go. He grabs his body, cupping one hand to his crotch as he hops comically on his uninjured foot.

The dancers around them alternately shriek, gawp, or flee as Sherlock sweeps into the fray, yanking Gary's arms behind him as he staggers on jelly-legs. The house lights come up, dazzling bright. He blinks stars from his eyes as Lestrade charges forward to help Molly to her feet, leaving him to frog-march her assailant off the dance floor.

Donovan takes no chances. Although Gary has clearly been incapacitated by Molly's self defence measures, she allows Sherlock to keep a grip on the injured man's scruff until she cuffs his hands far more securely than PACE guidelines allow. A burly bouncer hovers nearby. She nods her permission to advance and together they head for the manager's office. It is clear that Donovan has seen the glint of madness that illuminates Gary's eyes.

"I'm all right," Molly wheezes, although in the excitement, Sherlock hadn't bothered to ask after her.

Remembering belatedly that Molly is a pathologist and not a trained adventurer, Sherlock slips his arm around her shoulders. "You did just fine. Better than fine." He gives her a brief squeeze and a smile filled with pride. It's obvious she's putting on a show of bravado to cover her nerves, so instead of following behind Lestrade and Donovan, he guides her to the bar and gets her another drink – double vodka this time, no ice or soda – because he suspects that her part in the play might not yet be concluded and she's going to need the bracer before the curtain rises on the final act.

"Why did he call me Linda?" Molly asks as she regards her hands with a perplexed expression. "I thought the missing woman's name was Emily."

"I suspect," Sherlock replies with a cold sense of certainty, "he's fixated. Reliving some past relationship that went wrong, over and over again. It's possible when they dig up Gary's back garden, they're going to find the corpse of more than one woman."

The barman returns, putting glasses and a bottle from the reserve stock down before them. "On the house."

Sherlock absently nods his thanks as he considers the available data and then he supplements his hypothesis with a caveat. "Or if not, then they'll find evidence that at some point there was a Linda in his life, and that relationship didn't end happily. Whoever she was, she's caught your dance partner in a long shadow and he can't or won't step away from it."

He contemplates the drink in front of him and then downs it in one go. No doubt there's an object lesson about obsession to be had in the course of the night's events, if one bothers to look for it. But he doesn't. He'd like to think that there's no reason to compare his longing for what can't be to the compulsive fixation of a madman.

Two constables enter and head straight for the manager's office. A few moments later, they exit again, suspect in custody, trailed by Lestrade and Donovan and a clutch of mobile-wielding club-goers firing off their cameras apps. No doubt a few of them will manage to sell their out of focus photographs to the tabloids. Gary looks over at Molly. Tears roll down his cheeks and then he begins to sob openly as he tries in vain to reach for her one last time.

Molly flinches. Lestrade sees. He stops and drops out of the escort party long enough to dig into his pocket and hand Sherlock the key to his car. "You make sure she gets home safe. Then tomorrow I want to see the pair of you down at the nick. Okay?"

"Yes, Dad." Sherlock replies flippantly, even though he had already decided that he was going to take Molly home by taxi. He was, after all, in his own way a gentleman and she deserved that much.

Something like a combination of resignation and despair passes over Lestrade's face before he turns his attention to Molly. "I'm sorry about what happened to you. We were just hoping to spark a name. We had no idea..."

"I know." Molly shudders, a residual reaction, or perhaps a delayed one. "At least now her family will have some closure."

Lestrade holds her gaze for a long moment, and then he hurries to catch up with the others, but not before giving Sherlock one more pointedly significant look.

Sherlock glances around him. The club patrons have gone back to doing whatever it was they were doing before the excitement kicked off, only now, since there is an additional frisson of danger in the air, they seem to be doing it more intensely; dancing a little closer, holding one another a little tighter, giving their partners meaningful looks that seem to say, 'Carpe diem, because who knows what's coming next.' He reaches over and unpins the auburn wig from Molly's head and pulls it, and the little cap that holds her own hair, free. Suddenly, she is once again Miss Molly Hooper, his friend and colleague, albeit a slightly dishevelled version. She looks weary and seems ready to exorcise the ghosts of the dead and their mad lovers. "Take off those ridiculous shoes," he says as he offers his hand, "and come dance with me."

Molly cocks her head uncertainly, but after a moment she complies. The DJ spins a slow song and they rock together comfortably, celebrating a difficult job well done, content to leave whatever might come next between them in tomorrow's hands.