Blackfriars Station: where the Circle and District Lines run together, where trains shriek to a stop before pulling away, where the passengers move when the carriages do not. Also where John finds himself on a Tuesday morning, chasing after Sherlock as he runs down to the eastbound platform, taking the steps two at a time. He slips through the doors of the train just as the warning tone begins, John barely making it in before they slam shut. He slumps down onto a seat, catches his breath. Accidentally presses his leg against Sherlock's.

"What the hell are we doing?"

"Disproving an alibi. We have the records from our suspect's Oyster card saying that she tapped in at 11.23 and out at 11.44, but our victim was lacerated near Liverpool Street at 11.41, watch broken. The train leaves Blackfriars at 11.26 if there are no delays, which there wasn't. Just enough time for our suspect to jump the barriers, murder the woman, and come back to tap out."

"Couldn't we just look at the CCTV?"

"Mysteriously vanished. Incompetent."

"Awfully convenient."

There's a rumble underfoot, the whine of the engine increasing in volume as the carriage begins to move.

The next station is

Mansion House

Please mind the gap between the train and the platform.

He stares out the window, a rolling parallax of black wires on black walls. Sherlock's texting, his elbows encroaching upon John's chest as his fingers flit over the keys. Stations sweep by; at each, the train pauses, then speeds up with a quiet howl, wheels clattering against the track. Passengers stream in and out, a bearded man at Cannon Street, a short-haired woman at Monument, Tower Hill: tourists. The rocking of the carriage; rhythmic, lulling as it pushes the two together, apart.

This is

Aldgate

Mint and turquoise tiled pillars, boasting red and blue roundels. Doors open – eight minutes – doors shut.

This is a Circle Line train via

Liverpool Street and

Kings Cross/St. Pancras.

The next station is

Liverpool Street

Change for

the Central Line and

National Rail services.

The platform pulls into view, clusters of travellers with their heads down, buried in a phone, in the papers, in their own thoughts. A westbound train passes by in a blur of red and white, rattling away as soon as it arrives. Ten minutes.

"If she took this train, that leaves...five minutes between her exiting the train and the murder. Enough?" asks John.

Sherlock has already dashed out.

When John finally catches up, he finds Sherlock at the entrance of the station, beaming.

"What'd you reckon?"

"Undoubtedly her. Timing works out perfectly." He reaches for his phone, sends a text to Lestrade.

Arrest the suspect. SH

"Dull. This wasn't even a six. Well, they can't all be brilliant."

"Why'd you come, then?"

Sherlock doesn't reply, only smiles faintly. Then he turns and heads for a coffee shop.

"Black, two sugars, thank you. And tea for my friend."

He manoeuvres into a table by the corner, his back to the wall and the entire shop within his view. John picks up their drinks and sits across from him, pen scratching across a notepad as he unravels the details from their latest case. He sips his tea, tuning out the other customers as he outlines a narrative. Sherlock's phone chimes, eliciting a smirk and a glance upwards.

"What do you say to a riverboat ride?"

They arrive at Embankment Pier, step on an eastbound route to North Greenwich. The boat passes under Waterloo Bridge, a hulking concrete beast looming above them as the dome of St. Paul's rises alongside the towers of the City, leafy trees on both banks. Back to Blackfriars, gliding between its two and a half bridges, ruby columns upholding naught but air. The twisting coils of Millenium Bridge, pointing to the Tate Modern. Sherlock peers out onto the river when they reach the Tower of London, slipping from the sleek and modern to the historical, the antiquated. At Tower Pier, Sherlock lightly touches John's arm as he moves to the exit, typing something into his phone. Ahead: Tower Bridge in all its stone glory, azure and royal blue.

"There's been a murder at the Tower Hotel. Lestrade says our dead man took a river bus back to the hotel, then locked himself in the bathroom. Body wasn't discovered until the housekeeper entered," says Sherlock.

"How'd he die?"

"Drowned in the tub. No signs of foul play, and yet..."

"Maybe he fell asleep?"

"Perhaps. Best not develop baseless theories."

They enter the foyer of the hotel, all angles and geometry. Lestrade greets them, then takes them up to the room. There's a king-sized bed, unmade. By the window, a small seating area: two armchairs and a circular table. An open suitcase lies at the foot of the bed, clothes neatly folded within. On the nightstand: keycard, phone, a book, tickets.

"So. We've got a dead man, a barred room, and no motive nor suspects. Suicide?" asks Lestrade.

"Doubtful. There are simpler methods." Sherlock enters the bathroom, examining the tableaux.

Early thirties, tanned.

No ring, neatly trimmed nails; clean.

Relatively fit.

No visible injuries.

Recent insomnia.

Nearby towel: slightly damp.

Other towels: dry.

Faint imprint of a hand wrapped around faucet; not his, slightly larger.

"Someone covered his face with a towel and pushed him into the water. He was already partially sedated when he entered the tub – he'd taken sleeping pills to deal with jetlag – so it wouldn't have been much of a struggle. If it was murder..."

He shifts toward the door and inspects the handle, letting out a short laugh.

"John, will you stand inside? Now, turn the lock for me, please, and leave the door open." He exits the room and shuts the door behind him, confining John inside.

"Locked," he says, tapping the handle. "The murderer probably hoped – in vain, might I add – that it would deter people from entering. Clearly, not someone very experienced. You can open the door now, John." He moves to the centre of the room, scrutinising every corner.

"There's been a woman here," he says, pointing at the bed. "Two impressions in the pillows, yet he's unmarried and there's one suitcase. However, the murderer was a man. Surely you can deduce the rest."

"Um. Jilted husband?" asks John.

"Yes!" Sherlock exclaims, taking John's hands in his. "Finally, he understands." His eyes are gleaming as he collects the mobile from the nightstand, re-entering the bathroom. Comes back out, triumphant.

"Here's her number. He's yours," says Sherlock, handing it to Lestrade.

"There's one thing I'm still not clear about. Why would he ride the Clipper?" says Lestrade.

"Obvious. He's a tourist. Chinese or Italian?"

"Pardon?"

"For dinner."

A drink and a plate of fried rice later, Sherlock instantly reverts back to lethargy, stretching his legs underneath the table as he yawns. He catches John's calf with his foot, who gives him a long-suffering smile and returns to his writing. A bill is received, two fortune cookies piled atop it.

"You're about to ask if I can predict the fortunes," says Sherlock.

"Actually, yeah. I was. Can you?"

He picks one up, staring at it for a few seconds before setting it back down.

"Mmm. This one here, something inane about being proactive. The other one, love." John cracks open both, then hands one to Sherlock at random.

His says, Life is about making some things happen, not waiting for something to happen.

Before John can fully consider the implications of his own – Love, because it is the only true adventure – he looks back at his notes.

Then, a revelation.

"Sherlock."

"Yes?"

"Did you...use a fingerprint to unlock the mobile?"

"So what if I did?"