Season 2

1. A Scandal in Belgravia

Moriarty chooses to let Sherlock and John go after receiving a phone call. Some time later, Mycroft hires Sherlock and John to retrieve compromising photos of a minor royal, which are held on the camera phone of Irene Adler ( Lara Pulver ), a ruthless and brilliant dominatrix who also trades in classified information extracted from her rich and powerful clients. Sherlock obtains Adler's phone, but discovers it is booby-trapped and requires a code to disarm it. When Adler discovers that the CIA are on her trail, she disappears and is then apparently killed, only to reappear to ask John to get her camera phone back from Sherlock.

Weeks later when the coast is clear, Adler tricks Sherlock into deciphering a coded message on her phone which she obtained from another well-connected client. She sends the message to Moriarty, who in turn uses it to foil a British counter-terror operation. She almost succeeds in blackmailing Mycroft, but Sherlock finally cracks the password for the phone, leaving Adler without the protection she needs to survive. The episode concludes as Mycroft tells John that she has been killed by a terrorist group in Pakistan, while in fact she was secretly rescued by Sherlock. Based on the short story " A Scandal in Bohemia ".

2. The Hounds of Baskerville

Sherlock and John are contacted by Henry Knight ( Russell Tovey ), a man traumatised by the death of his father by a monstrous hound on Dartmoor years before. Investigating Dewer's Hollow, a local spot where the beast was allegedly seen, as well as the nearby Ministry of Defence testing site Baskerville, Sherlock and John uncover a conspiracy wherein one of the Baskerville scientists, Dr. Frankland ( Clive Mantle ), is continuing the work of H.O.U.N.D., an aborted project to create a hallucinogenic gas for military use. Sherlock and John discover that the legendary hound is an ordinary dog used for publicity that the hallucinogenic gas makes appear as a demonic monster.

The "hound" that killed Henry's father was actually Frankland wearing a red-lensed gas mask and a T-shirt bearing the logo of the H.O.U.N.D. group. Confronting both the dog and Frankland at Dewer's Hollow, John and Lestrade shoot the dog. Frankland attempts to flee, but dies when he runs into a minefield. In the final scene, Mycroft releases a confined Jim Moriarty. Based on the novel The Hound of the Baskervilles.

3. The Reichenbach Fall

Moriarty launches a simultaneous heist on the Tower of London, Bank of England, and Pentonville Prison, crimes for which he allows himself to be captured and put on trial. He secures not-guilty by intimidating the jury and visits Sherlock, explaining he still intends to "burn" him, taunting him with a "final problem" for him to solve. He also tries to destroy Sherlock's reputation. The two meet on the roof of a hospital, where Moriarty explains that assassins will kill John, Mrs. Hudson, and Lestrade if Sherlock does not commit suicide; he wants Sherlock to do this to cement his story after explaining that his "god code" was a myth.

Moriarty kills himself to force Sherlock to do the same. Sherlock calls John, confesses to being a fraud, states his final "goodbye" and then steps off the roof. Inspired by the short story " The Final Problem " and makes references to " The Greek Interpreter " (with the introduction of Mycroft Holmes at the Diogenes Club ) in addition to other works of Sir Arthur Conan Doyle. The title alludes to the Reichenbach Falls, where Holmes and Moriarty supposedly fall to their deaths in the original story.