White, however, has the move, and the move in this case means the initiative, and the initiative, other things being equal, is an advantage. -José Raúl Capablanca, Chess Fundamentals
The case began with a movie.
Or more accurately, it began when John was shamelessly begging for one of the pies Mrs. Hudson had just baked. Sherlock was sitting in the corner, taking frequent breaks from an abnormal psychology text to shoot disgusted looks at John. But damn it, he was starving and hadn't enjoyed a proper meat pie since his mum died.
"Not your housekeeper, dear. Besides, they're nothing much, just trying to use up the last of that roast before it got dodgy. You wouldn't want them."
"They smell delicious," John wheedled. "It's not as if you were Mrs. Lovitt, I know I can trust your beef."
And that did it; she laughed and sent him away with two of the hand-size pies. It was a test of John's self-control, but he managed to eat only one and set the other in front of Sherlock, who actually ate several bites before he fell to digging out pieces of meat and teasing them apart with his fork. It was like eating with a child, honestly.
"Who's Mrs. Lovitt?" Sherlock asked, and John could see from the look in his eye that the question had been burning in him all afternoon. He had obviously heard the name and suspected some yet-undiscovered connection between his flatmate and his landlady.
John stopped licking the gravy off his fork and stared. "You mean you haven't seen- Of course you haven't."
So when there was another afternoon with no case and no work at the surgery and it was pissing rain again because they lived in bloody London, he told Sherlock, "Right, we're going to watch Sweeney Todd."
"What?" Sherlock was only half paying attention, perched on the sofa with John's laptop balanced on his knees.
"Film about an English serial killer. You'll like it."
"I doubt it," Sherlock said darkly, the hint of a sulk on the edge of his voice. John was used to this and ignored it, heading upstairs to get the DVD from his room.
"Sweeney Todd is a fictional English serial killer," Sherlock announced upon John's return. "That's why I hadn't heard of him." He sounded aggrieved, like he suspected John had tried to play a trick on him.
"How did you know that, then?"
"Googled it." Sherlock was busily typing again. John rolled his eyes and stacked a few of Sherlock's larger books on the coffee table to make a platform.
"Just try it, Sherlock. You liked Seven."
In his efforts to find something in popular culture that would derail Sherlock's burgeoning obsession with talk programs, John had begun showing him films about clever or unusual murders. He wondered a bit at what this plan said about his own sanity, but really anything was better than having to watch repeats of Jeremy Kyle in freeze frame so Sherlock could pinpoint "the exact moment at which she realizes he cheated on her, look John!"
"Ye-es," Sherlock conceded. "But that one with the pedantic cannibal was ridiculous." Sherlock also had not been impressed with Saw, but to be fair neither had John. He'd really liked Psycho, had spent an entire afternoon researching Ed Gein on the internet instead of shooting holes in the wall or whinging, and John had been planning to show him Zodiac next. But Sweeney Todd, come on.
John snapped his laptop out of Sherlock's hands and set it firmly on the book-platform. Sherlock made a protesting noise, but didn't stir toward his own laptop, which was half-hidden under the coffee table. "Well, what's it about then?" he said grudgingly. John had learned to hold these exercises in cinema appreciation on days when Sherlock had just come off a case and was therefore feeling sated and indulgent, not yet succumbing to boredom.
John thought for a moment. "It's like The Count of Monte Cristo, but with serial murder." Sherlock looked blank. "Alexandre Dumas? Didn't you ever study literature?"
"I did but-"
"I deleted it," they both said together. Sherlock frowned in annoyance.
John popped the DVD in the drive and clicked his way into the video player. "We're watching this," he said decisively. "Besides, it has Johnny Depp. Everyone likes Johnny Depp."
"Only because you don't know who he is." John clicked play and flopped onto the sofa next to Sherlock. Evidently it was a hit, because Sherlock was relatively quiet for the first thirty minutes or so. (Other than an agonized, "Oh god, it's a musical.") The restraint lasted until Judge Turpin showed up at Todd's barber shop, and Sherlock began ranting, "Why does no one in this ridiculous film recognize anyone else?" Sherlock's mobile went off at that point, and he ignored it while it rang four times, which was a clear sign that he had become interested in the film despite himself. Then John's mobile, upstairs in the bedroom, rang four times. Sherlock's mobile immediately went off again, and Sherlock scrabbled in the sofa cushions for it, because that was Lestrade's signal for Pick up the phone, you lazy sod.
John hit pause and closed the laptop while Sherlock answered, because he knew from experience that it would be less than five minutes before Sherlock was fully dressed and ready to whirl out the door on whatever case Lestrade had dug up.
"Ah," Sherlock sighed in bliss, pocketing his phone. "We have another serial murderer. A non-fictional one."
"Best put this away then," John said, sticking the DVD back in the case. "I expect you won't have time to look at it for a while."