In the wake of their terrifying poolside face-off with Sherlock's insane arch-nemesis Moriarty, John blogs, Sherlock deduces, and the cases — and pageviews — roll in. But when a mysterious summons transports John and Sherlock to the inner chambers of Buckingham Palace, Sherlock's peevish brother Mycroft, an enigmatic government agent with "ultra" high-security clearance, explains that he needs Sherlock's help. Irene Adler, professionally known as "The Woman," has compromising photos of a young royal on her smartphone. Adler has reached out to the royals, although not to extort money or favor. A power play, Sherlock observes, enticed, and so begins his adventure with the woman whose website advises, "Know when you are beaten."
Sherlock and John visit Adler's Belgravia home, where she has prepared a greeting that would bring most men to their knees. The phone, whose contents are locked behind a passcode impenetrable even to Sherlock, soon becomes a dangerous liability to the inscrutable, incredible woman. Sherlock is left not just with the mystery of the passcode but that of his own feelings. So intense are those mysteries, and so high are their stakes, that John and even Mycroft fear this just may be the case to actually beat Sherlock.
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Warning: Contains significant plot spoilers
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Warning: Contains significant plot spoilers
Poolside, picking up from the season 1 cliffhanger mid face-off with arch villain Moriarty, Sherlock and John wait, assassin's sights trained on them. Sherlock aims his gun at Moriarty, then lowers it to the vest of explosives on the floor between them, ready to shoot. But Moriarty's phone suddenly rings, and the conversation changes his mind. "Sorry, wrong day to die," he tells Sherlock, before departing.
During an investigation, John and Sherlock are abruptly transported to Buckingham Palace. There, Mycroft and a palace official explain that a member of the royal family has had compromising pictures taken by the dominatrix Irene Adler. Sherlock and John visit Adler's Belgravia home where the dominatrix greets him naked but for a pair of heels. She knows who he is and that he's been to the palace. What's more, she knows he's after the phone, and she won't give it up. Sherlock is able to deduce the location of Adler's safe, and the dominatrix teases Sherlock that she's already told him its combination code. But an armed group of CIA operatives suddenly storms into the room. Their leader threatens to kill John if Sherlock doesn't open the safe and give them the phone. Sherlock claims that he doesn't know the code but at the last moment exchanges a poignant look with Adler and opens it, triggering a gun from the inside, which kills one of the men. Together, they disarm and knock out the others. Adler warns Sherlock that she will die before letting him have her phone. She injects him with a sedative and tells John that Sherlock deduced the code from her measurements.
Sherlock awakens in his bedroom with a very suggestive text alert programmed into his phone, courtesy of Adler. Mycroft abruptly orders Sherlock away from Adler but the dominatrix remains in touch. On Christmas, she texts, "Mantelpiece." There, Adler's phone sits, wrapped up as a Christmas present. Sherlock informs Mycroft that they will find Adler dead, soon confirmed at the morgue by the corpse of a woman identical, but for an unrecognizably damaged face, to Adler.
Distraught, Sherlock observes that John's blog counter is stuck at 1,895 and enters the number into Adler's phone as a passcode, but the screen displays "I am locked. Wrong passcode. 3 attempts remaining." Meanwhile, John is picked up by an anonymous woman in a limo, Mycroft-fashion, and taken to the Battersea Power Station. But it's Adler who meets him, explaining that she needed to fake her own death; she sent her phone to Sherlock for safekeeping; and she now needs it back. John pleads with her to let Sherlock know she's alive, so she texts him. They hear the signature alert and know that he's been there the whole time.
Sherlock renews his focus on Adler's phone, x-raying it, attempting another passcode (2 2 1 b), all to no avail. Before long, Adler shows up at Baker Street to retrieve her phone, explaining that it holds extremely dangerous information that she doesn't understand, stolen from a Ministry of Defense client. Sherlock offers to help and effortlessly breaks the code, revealing its reference to seats on a 747 scheduled to leave Heathrow airport the next evening. Sherlock digs further, connecting this flight, 007, to a statement Mycroft had made on the phone in his presence: "Bond air is a go. Check with the Coventry lot." But Adler secretly texts the revelation to her contactÉMoriarty! Moriarty in turn texts Mycroft, revealing that he is now aware of the Ministry of Defense plot to dupe a terrorist cell attempting to blow up a flight. As Adler attempts to seduce Sherlock, he sources Mycroft's reference to the famous WWII Coventry code break and solves the mystery: the Ministry has learned of a terrorist plot to blow up a passenger jet. Rather than exposing the source and possession of their intelligence, they have chosen to let the terrorist plot go forward.
But only when Mycroft again collects Sherlock, delivering him to flight 007 sitting on the tarmac at Heathrow, does Sherlock learn the neat solution that Mycroft devised: They had indeed intended the flight and bombing to go forward. But to save lives, they filled the plane with corpses, so that of the hundreds of reported casualties, no one is killed. Now, because Sherlock broke the code and Adler shared the information with Moriarty, the criminal mastermind was able to inform the terrorists that they and their plot are known. Adler joins the brothers on the plane and threatens to topple the entire government with the contents of her phone unless Mycroft meets her requests. She tries to humiliate Sherlock about his having believed in her sincerity. But Sherlock actually holds the power; back at 221b Baker Street, he'd taken her pulse. He'd observed her dilated pupils. And he knows the phone's passcode: "I am S H E R locked." He hands the unlocked phone to Mycroft and departs.
Some months later, Mycroft tells John that Adler was captured by a terrorist cell in Karachi and beheaded. They plan to spare Sherlock by telling him she's in a witness protection program in America. But only Sherlock knows the truth: her terrorist executioner was actually Sherlock, who, disguised, rescued her as he wielded the sword.