A Scandal in Belgravia

Jim Moriarty, whose snipers are aimed at Sherlock Holmes and Dr John Watson, and who has stated his intention to kill them both, is interrupted by a phone call. He leaves, having "received a better offer", letting Sherlock and John return to their flat at 221B Baker Street.
Sherlock Holmes solves a number of cases over the next few weeks, turning down several others because they bore him, including a man claiming his aunt's ashes are not hers and two girls not allowed to see their dead grandfather. Sherlock becomes a minor celebrity following John's blogs about his activities. One day, Sherlock's brother Mycroft has the pair brought to Buckingham Palace for a meeting. Mycroft and a Palace official explain that a female member of the royal family has had compromising photographs taken with dominatrix Irene Adler, and that she wishes for them to be retrieved. While Sherlock reviews photos of Adler, who is referred to as "The Woman", she looks at pictures taken of him.
Sherlock and John visit Adler's home, attempting to use deception to get inside. However, Adler is expecting them and after considering possible outfits, she appears fully made-up but completely naked. Sherlock is consequently unable to deduce anything about her. After several rounds of banter between Sherlock and Adler, John sets off the fire alarm and Sherlock is able to determine the location of Adler's safe, where a camera phone containing the compromising photos, as well as other valuable information, is hidden. Several American operatives appear and hold Sherlock, John and Adler at gunpoint, demanding that Sherlock open the safe. Sherlock deduces the password (Adler's measurements) and opens the safe, which is booby-trapped with a handgun and kills one of the assailants. They disarm the rest of them. Sherlock acquires Adler's camera phone, but she attacks him with a drugged syringe and escapes through a window with the phone. During this time, John is in another room. Back at his apartment, Sherlock has a fevered dream in which he sees Irene returning his coat. He wakes to find that his coat has mysteriously reappeared, and Adler has added her number to his phone. She has added a personalised ringtone of a woman sighing erotically, which sounds whenever Sherlock's telephone receives a text message she has sent.
Six months later, whilst celebrating Christmas, Sherlock learns (via text message) that Adler has sent him the camera phone for safekeeping. Sherlock notifies Mycroft that he will shortly find Adler dead, knowing the value Adler put on her phone. Subsequently, the Holmes brothers arrive at St Bart's morgue where Sherlock identifies Adler's mutilated body. Some time later, John is contacted by an unnamed woman who has him brought to the abandoned Battersea Power Station. The contact reveals herself to be Irene, not Mycroft as John expected: she faked her own demise to shake pursuers off her trail. John urges her to reveal herself to Sherlock. She initially refuses, but is persuaded. The two then discuss the nature of their respective relationships with Sherlock; during this, Adler's personalised ringtone is heard from an adjacent corridor, revealing Sherlock's presence and his awareness of Adler's survival.
Back in 221B, Sherlock detects the signs of a break-in and finds the American hitmen from Adler's residence holding Mrs Hudson hostage. Sherlock immediately realises that the landlady has been tortured and plans extreme retaliation. During the stand-off, Sherlock demands two of the gunmen go away, leaving him with their leader. While the man frisks him, Sherlock knocks him out cold, ties him up, and eventually throws him out the window.

Adler kissing Sherlock to encourage him to crack the code.

Later on, Sherlock finds Adler sleeping in his bedroom. Irene reveals that she is still being hunted and asks Sherlock to decipher a code she stole from a Ministry of Defence official. He effortlessly cracks the code, revealing it to be an airline seat allocation number. Irene secretly texts the flight number to her contact, Jim Moriarty. He in turn texts Mycroft Holmes, revealing that he is now aware of the MoD ploy to fool a terrorist cell that was attempting to sabotage the flight. Mycroft is visibly shattered by this development.
Adler's attempts to seduce Sherlock are interrupted by government officials who have come to collect him and deliver him to Heathrow airport. En route there, Sherlock remembers Mycroft mentioning Coventry on the phone and reminisces about the allegations that the British government allowed the Coventry Blitz to happen, so as not to alert the Germans that their military codes had been cracked. There, his suspicions that a similar situation is occurring are confirmed by Mycroft on board the airplane, which has been filled with corpses. The government had decided to fly a 'dummy plane', so as not to alert the saboteurs yet still avoid genuine casualties, which also explains the involvement of US agents. However, as Sherlock has unwittingly helped Irene, and, by extension, Moriarty, crack the code, the scheme is foiled.
Afterwards, the Holmes brothers and Adler sit down, while Adler reveals a list of demands, including protective measures for herself, against the release of further confidential material. Mycroft is helpless; however, at the last moment, Sherlock confronts Adler, deducing the password of her camera phone. Despite Adler's cold insistence that Sherlock meant nothing to her, taunting his lack of sexual or romantic experience, he has already concluded she is lying. He points out her dilated pupils and her elevated pulse during moments of intimacy – both indicators of her own attraction to him. He types S-H-E-R into the phone, which reads "I AM S-H-E-R LOCKED". Passing it to his brother, Sherlock walks out, ignoring Adler's pleas for protection. Without the insurance of the information she held, she is unlikely to be able to outrun her enemies.
Some months later, Mycroft informs John that Adler has been beheaded by a terrorist cell in Karachi, but asks him to tell Sherlock instead that she has entered a US witness protection program. Sherlock appears to accept this and asks John to give him Adler's phone. John leaves, while Sherlock re-reads her multiple flirtatious text messages to him. The scene changes to a flashback of Adler's "execution": Sherlock had infiltrated the cell, disguised as her executioner, and rescued her with seconds to spare.