Mycroft needs Sherlock's help, but a remorseless criminal mastermind puts Sherlock on a distracting crime-solving spree via a series of hostage human bombs through which he speaks.
Mycroft wants his brother Sherlock to investigate the murder of civil servant Andrew West and theft of the Bruce-Partington missile plans. Sherlock, however, prefers to assist Lestrade following a massive bomb blast. The sadistic bomber abducts a string of hostages, who will be released after Sherlock has solved a series of puzzles, including a twenty year old murder, an insurance scam, and the alleged forgery of a long lost painting.
Sherlock's brother Mycroft presents him with a case of utmost importance to the country and the government. While proceeding to solve that case, Sherlock encounters difficult distractions from a mysterious villain who abducts people and threatens to kill them unless his demands, which are meant for Sherlock, are met. Dodging these distractions, Sherlock solves the original case but comes face to face with the deadly intelligent villain, Sherlock's archenemy, Professor Moriarty!
Sherlock's boredom ends with a bang, thanks to a bomber targeting the edifice facing 221B Baker Street. Mycroft approaches Sherlock with the death of government employee Andrew West and his missing copy of the Bruce Partington plans, but a soulless criminal mastermind, speaking through a series of human bombs (innocent Londoners wired to explode), distracts Sherlock, sending him, Watson and Lestrade scurrying through the city to solve a 20-year-old masked murder, an insurance fraud business, a celebrity poisoning (murder), and assassinations surrounding a forgery scheme. Someone is playing with Sherlock, and he rightly suspects it to be his brilliant criminal equal, the mysterious, unseen figure known only to him as Moriarty.
Sherlock Holmes is bored - there are no intriguing murder cases to solve. Then a nearby flat is blown up and Holmes starts to think something may be afoot. His brother Mycroft visits and tries to get him to solve a suspected murder case involving an MI6 officer and find the top secret missile plans he had. Holmes is not that interested. The bomb case gets more intriguing, and deadly, when Holmes is contacted by a would-be victim of the bomber. Unless Holmes solves a certain case within 12 hours, the bomb attached to the person will be detonated. Time is of the essence.
A bored Sherlock Holmes suddenly has two cases to keep himself occupied. His brother Mycroft approaches him to investigate the death of civil servant Andrew 'Westie' West, whose body was found in the train yards. More importantly, Westie had a USB stick containing the detailed plans for the Bruce Partington missile defense system which cannot be found. Somewhat to Dr. Watson's surprise, Holmes refuses to even respond to his brother's calls or texts; so, out of embarrassment as much as anything else, the good doctor decides to investigate the case himself. Holmes is far more interested in solving DI Lestrade's case of a mad bomber who is spreading fear throughout the city by forcing innocent individuals to act as bombers unless Holmes can solve the puzzles he's presenting to him. What Holmes comes to realize is that both cases are related and he will soon face his arch enemy.
- Sherlock Holmes is bored and lacking a case, and in frustration he keeps shooting his gun at the wall in his flat. John Watson, while finding Sherlock impossible to be around, spends the night at his doctor/girlfriend's flat, and sees news of an explosion at Baker Street on the television. He rushes home and finds Mycroft pressuring Sherlock to investigate the murder of an MI6 clerk and the disappearance of a flash drive with important defence plans. Sherlock refuses.
Sherlock is called to Scotland Yard. Inside the bombed-out flat was a strongbox containing a pink mobile phone similar to the one belonging to the victim from "A Study in Pink". A message on the phone leads Sherlock to a pair of trainers in a basement and then receives a call from a terrified woman, who is obviously reading a message from an unknown third party. If Sherlock does not solve the puzzle in twelve hours, the explosive vest she is wearing will be detonated.
While Sherlock and John examine the trainers back at the lab they are interrupted by Molly Hooper, who is excited to introduce her new boyfriend Jim. Hes an IT employee, but Sherlock isnt impressed and he immediately deduces Jim is gay, and Molly storms out in anger and embarrassment. Sherlock traces the shoes to a schoolboy named Carl Powers, who drowned in a pool in London some years ago. Sherlock, who was underage at the time, was unable to convince the police to take his theories seriously, but he proves that Powers was poisoned via his eczema medication. And now that Sherlock has solved this particular case, the booby-trapped woman is freed.
A second message shows a sports car stained with blood and the man who rented it has disappeared, and another hostage tells Sherlock hes got only eight hours to solve this mystery. Sherlock has a brief but revealing chat with the missing mans wife and then he interviews the owner of the agency that rented the car. He deduces that the man has a distinct suntan and was recently in Colombia, although the man claimed hed not been anywhere and the tan was only from a sunbed. Finding that the blood in the car had been previously frozen, Sherlock concludes that the lost man Ian Monkford and his wife, paid the agency owner to help him disappear because he was probably facing huge debts. So once again after solving this case, the hostage is freed.
A third message and a third hostage points Sherlock to the death of television personality Connie Prince, apparently from tetanus. Supposedly she cut herself on a nail, but it seems that the wound was made after her death. Sherlock pins the crime on the housekeeper (also her brother's lover), who murdered Prince by increasing her botox injections. Although Sherlock solves the puzzle within the twelve hour time frame, the bomber triggers the explosives when the hostage, who is blind, starts describing her kidnapper's voice. The blast kills her and eleven other people in her building.
The fourth message is a photograph of the River Thames, though no hostage calls this time. Sherlock finds the corpse of a security guard on the riverbank, identifying the murder as the work of "the Golem", an assassin. Sherlock traces the Golem through his network of homeless persons, but hes too late to stop the murder of an astronomy professor whom the guard had talked to. They both realised that a recently discovered painting by Vermeer, which was valued at around £30million, was a fake. While Sherlock is examining the painting at the museum, the fourth hostage calls and its a childs voice this time, giving Sherlock only a long ten second countdown to prove the painting is a fake. He does so, and the museum curator admits to faking the picture, and that her secret "partner" was called Moriarty.
Investigating Mycroft's case in secret, Sherlock and John trace the MI6 clerk's death to his prospective brother-in-law, who confesses that he stole the flash drive and accidentally killed him and planted his body on the top of a train. The man stole the memory stick because of his financial troubles, but still has it because he had no idea how to sell it.
Sherlock waits for John to go out and then arranges a secret meeting with Moriarty. They meet at the nearby indoor swimming pool where the boy Carl Powers had drowned, but he is met instead by John who is wearing one of those explosive vests. Moriarty appears and introduces himself, and he turns out to be Molly's boyfriend Jim (from earlier on in the episode). Its clear now that he was just pretending to be accident prone and gay at that time, in order to put Sherlock (and the audience) off the scent. Moriarty tells Sherlock its been fun playing these games but now he had better back off and stop interfering with him, but Sherlock refuses. Moriarty leaves momentarily and Sherlock manages to get the explosive vest off John and slide it away. But then he comes back and tells them both that he cant let them live, and multiple sniper targets appear on the two men. Sherlock aims his handgun at Moriarty, but then changes his aim to the explosive vest, which is now next to where Moriarty is standing across the pool deck.
The episode (and the first series) ends on this cliffhanger.