John Watson, an ex-army doctor injured in the war in Afghanistan, is invalided back home to England and is looking for a flat somewhere in London. A mutual friend introduces him to Sherlock Holmes because he knows they are both looking for somebody to flat-share with. After an uneasy first meeting, they do become flatmates at 221B Baker Street, a property which is owned by the landlady Mrs. Hudson.
There have been a strange series of deaths recently, which Inspector Lestrade thinks are serial suicides. Sherlock looks at the latest crime scene, which is of a woman wearing pink. Sherlock deduces that the woman is a serial adulterer with an unhappy marriage. Unlike the other victims though, she left a big clue by clawing "Rache" into the floor. Sherlock ignores the suggestion of the forensic expert Anderson, that it is the German word for revenge and settles on "Rachel", deeming that the victim died before finishing the scrawl. Sherlock finds splashes of mud on the woman's leg, thrown up by the wheel of a suitcase, and deduces that she is from out of town. The police found no suitcase on the premises, but Sherlock searches for it, later finding it in a nearby skip.
Meanwhile, John receives a call from a public phone and is taken to an empty warehouse. There, he meets a man who claims to be Sherlock's "arch-enemy". The man offers money in return for information about Sherlock's activities, but John shows loyalty to Sherlock and refuses. When John returns to 221B, Sherlock asks him to send a text message to the phone of the fourth victim.
While dining at a local restaurant with John, Sherlock notices a cab and gives chase. And while using his extensive knowledge of London's streets and alleys, he manages to catch up with it on foot. However, when he and John open the cab door, they find that the passenger is a newly arrived American: a perfect alibi.
Suspecting that Sherlock had withheld evidence, Scotland Yard executes a drugs bust on his apartment so that they can search it. Sherlock guesses that "Rachel" was the victim's mail address password and that the victim had planted her mobile phone on the murderer so that he could be traced by GPS. At the same time that John finds that the signal is coming from 221B, Mrs. Hudson tells Sherlock that there is a cabbie waiting for him downstairs and even though he didnt order one, he leaves the apartment and enters the cab. The cabbie confesses, but insists that he doesn't kill any of his victims, instead he speaks to them and they end up killing themselves. They arrive at a school building and the cabbie pulls out a gun and two small identical bottles containing pills. He claims that one contains a harmless pill and the other a poisonous pill, and that each victim has to choose and take one. He challenges Sherlock to solve his puzzle by choosing one of the pills.
Sherlock realises that the cabbie is dying and has no other reason to live, apart from his kids who we see a photograph of in his cab. The driver admits this and says that he gets paid by a sponsor for every one of these victims and the money goes to his kids. Sherlock calls his bluff on the gun which is actually a novelty cigarette lighter, and goes to walk out. The cabbie challenges him again though to choose a pill and see if he can beat him at his own puzzle. Meanwhile, John has traced the GPS signal from the victim's phone and followed Sherlock. Through a window in the adjacent building, John sees Sherlock about to take one of the pills, and shoots the cabbie. Sherlock questions the dying cabbie first about whether he got the pill game right, and then he asks for the name of his sponsor that the cabbie had mentioned. Finally, with his last words he shouts out the name "Moriarty".
Outside, Scotland Yard has surrounded the perimeter and Sherlock starts to make some comments to Lestrade about the shooter. He suddenly realises though that it must have been John, and then he feigns shock and asks Lestrade to ignore everything he has just said. Sherlock and John walk away from the scene but run into the man who had abducted John earlier, and he it turns out to be Sherlock's elder brother Mycroft Holmes. Mycroft instructs his secretary to increase their surveillance status, while Sherlock and John return to Baker Street.
- Benedict Cumberbatch as Sherlock Holmes
- Martin Freeman as Dr. John Watson
- Rupert Graves as DI Lestrade
- Una Stubbs as Mrs. Hudson
- Louise Brealey as Molly Hooper
- Vinette Robinson as Sgt Sally Donovan
- Tanya Moodie as Ella
- Siobhan Hewlett as Helen (as Siobhán Hewlett)
- William Scott-Masson as Sir Jeffrey Patterson
- Victoria Wicks as Margaret Patterson
- Sean Joseph Young as Gary (as Sean Young)
- James Duncan as Jimmy
- Ruth Everett as Political Aide
- Syrus Lowe as Political Aide
- Katy Maw as Beth Davenport
- Ben Rufus Green as Reporter (as Ben Green)
- Pradeep Jey as Reporter
- Imogen Slaughter as Reporter
- David Nellist as Mike Stamford
- Louise Breckon-Richards as Jennifer Wilson
- Jonathan Aris as Anderson
- Lisa McAllister as Anthea
- Stanley Townsend as Angelo
- Peter Brooke as Taxi Passenger
- Phil Davis as Jeff Hope
- Lasco Atkins as Late Night Pedestrian
- Alison Egan as Jimmy's Mum
- Mark Gatiss as Mycroft Holmes (uncredited)
Allusions to original Sherlock Holmes novels Edit
Episode connections Edit