Dr Culverton Smith is widely acclaimed as the greatest toxicologist in Britain.
Only Sherlock Holmes knows he's a murderer too.
The first victim is his young assistant, Victor Savage. No one is able to determine the cause of his death, but Sherlock knows better; Smith has been experimenting on toxins for years now, and the sadistic side of his personality has taken over at last.
Savage is the first of many, though Sherlock is unable to prove it. The great scientist is far too clever; of course the substance he uses can't be detected in post-mortem examinations, he would never risk being caught.
In the end Sherlock talks Molly into handing him all the autopsy records, and discovers that there are traces of injections on all the victims' bodies.
That night he breaks into Smith's house, opens his safe, and steals the phial that rests underneath a pile of documents. In the morning he heads to Barts with a triumphant smile on his face, determined to expose Culverton Smith as the criminal he actually is.
xxx
Molly twitches her fingers nervously as he wanders around the lab, running tests on the sample of the substance that has to be Smith's murder weapon.
"Are you sure of what you're doing, Sherlock? I mean, if you're right and this is a toxin of some kind, we should be more careful."
"Injections, Molly – remember? Culverton Smith injected the victims with the contents of this phial. As long as we don't accidentally touch it, we should be fine."
He's busy making complex calculations when his phone notifies him with an incoming text message. A cold hand of fear grips at his chest as soon as he reads it.
You always think you're the smartest. Injections were just a ruse. The toxin is lethal by inhalation too.
His eyes suddenly focus on Molly, who's staring back at him – anxiety written all over her face.
What have I done? he thinks, angry at himself as he's never been before.
His fingers tremble ever so slightly as he calls the only person who can possibly help them out of this mess.
xxx
They spend the next couple of hours huddled on the tiled floor, waiting for Mycroft to call back.
No matter how hard he tries, he just can't ignore the haunted look on Molly's face. The price for his arrogance is far higher than he ever expected; he wouldn't mind if he were the only one who's dying, but he's not, and he's never going to forgive himself for that.
Not that he has much time left to do so anyway.
"I'm sorry," he murmurs, his voice thick with guilt and regret. "You were right all along, I should have listened."
"It's okay," she replies softly, a watery smile tugging at her lips. "It's not your fault."
"It is. Moriarty didn't kill you, but I did."
Her hands are warm as they wrap around his own. "I'm not dead, Sherlock."
"Not yet."
"I've heard what you said on the phone. Culverton Smith surely created an antitoxin as well; your brother will find it."
"It might be too late," he counters, shaking his head in defeat.
That's when his phone rings.
xxx
Biohazard-trained personnel sweep them away to a disused facility where they can receive medical assistance. Mycroft isn't there, but then Sherlock never expected him to be.
"You should call John," Molly prompts as soon as they're alone once more.
"What for?"
"I don't know. Saying goodbye, I suppose."
He closes his eyes briefly, and for a moment there he's back on that rooftop – he can't do this again.
"What about you? Why don't you call your fiancé?"
It's the slight intake of her breath that gives it away. "Tom isn't my fiancé anymore. We broke up a week ago."
The signs are all there, he can see them now; he has no idea why he didn't notice before, he supposes his mind was too wrapped up in the Smith case.
"I'm sorry. He's a good man."
She blinks back the tears and looks away. "He is. It's just, he's not…"
You, that's what she leaves unspoken, but he hears it all the same.
Silence stretches around them until he can't bear it any longer.
"I don't believe in love," he whispers at last. "But if I did, you're the one I'd choose."
Next thing he knows, she's sobbing against his shoulder. Hard.
xxx
Mycroft shows up at the eleventh hour, and he's paler than his brother has ever seen him.
"Culverton Smith isn't an easy nut to crack," that's all he says before he hands a phial to one of the doctors.
"How did you persuade him?" Sherlock can't help but ask, though his voice is hoarse and his head is spinning.
"You don't really want to know, brother dear," Mycroft answers tightly. "He's not an issue anymore."
As a matter of fact he doesn't really care whether his brother tortured Smith, as long as they're still in time to save Molly's life.
His own life matters far less to him, except that he doesn't want to make his friends suffer again – and his parents too, he reminds himself before he eventually surrenders to exhaustion and sleeps.
xxx
He doesn't think he's ever been more relieved than the moment the doctors tell them they're going to be fine. Mycroft is waiting for them in the car, neither of them utters a word as the streets of London unfold before them.
Sherlock meets his brother's eyes for a moment, silently thanking him for what he's done. Mycroft only nods, then looks out of the window.
The car eventually stops at Molly's place; his gaze follows her as she gets off the car, unlocks the front door and disappears into her flat. It's a pity that she doesn't have anyone to look after her; he never really liked Tom, but she did, while she's all alone now.
Wearily he runs a hand across his face, waiting for Mycroft to give directions to the driver. His brother stares at him instead, then rolls his eyes in annoyance.
"Off you go, Sherlock."
He raises a quizzical eyebrow, but Mycroft doesn't buy it in the slightest.
"I'm not that dumb, you know," his brother reminds him. He all but shrugs, then steps out of the car.
(Later on, as Molly's arms are wrapped around his back and his lips return to the curve of her neck again and again, he has to acknowledge that – as much as he hates to admit it – his dearest brother is probably right.)