Here are summaries (warning—there are major spoilers) of episodes in season one.
Season One (2010)
“A Study in Pink” (Episode 1)
The police investigate the deaths of a series of people who all appear to have committed suicide by taking a poisonous pill. Sherlock Holmes joins the investigation and believes that the poisoning is the work of a serial killer and not individual suicides.
Sherlock meets Dr. John Watson, a war vet and Army doctor. Watson eventually moves into 221B Baker Street, along with Sherlock—the flat is owned by a widow, Mrs. Hudson.
Sherlock eventually finds himself face to face with a man who may be the killer and is challenged to a game of wits to the death. As part of the game, the killer puts two bottles on the table—one is harmless, but the other contains poison. Sherlock engages the killer in conversation and realizes that he knows the answer to the game.
As he prepares to take the pill that he believes to be harmless, a shot kills the man, who was a cabbie with a terminal illness. Just before the man dies, Sherlock asks the man who is really responsible, and he answers, “Moriarty.” Sherlock sorts out that Moriarty promised to send the man money to support his children for each successful murder. These murders were intended to attract Sherlock, who was the intended target.
Sherlock realizes where the shot came from and returns to Baker Street with Watson. There’s one final twist—Sherlock’s brother, Mycroft Holmes, arrives.
“The Blind Banker” (Episode 2)
Sherlock and Watson head to the bank, where an old university acquaintance of Sherlock asks for help. There was a break-in at the bank, and they want to pay Sherlock to find the hole in their security. Sherlock turns down the money, but Watson accepts.
Sherlock sees on the news that a journalist has been shot and killed in his locked apartment. Sherlock believes that the death may be connected to an earlier death linked to their investigation.
The thread in all of these crimes is graffiti symbols, which are ciphers of an ancient code. The key to the bank break-in and the murders appears to be a mysterious cipher. Sherlock consults a graffiti artist to learn more.
Sherlock and Watson crack the code—the numbers are in Hang Zhou, an ancient Chinese numeral system. They further realize that the code and the murders are connected to an illegal antiques smuggling operation.
Sherlock and Watson discover a key figure, Soo Lin, hiding at the museum. She explains that the code is linked to the Black Lotus Tong crime group, of which she was once a member. She further explains that he has a direct connection to the killer. She is later killed, as she feared she might be.
Watson and his date are kidnapped when the crime group mistakenly believes he is Sherlock. Sherlock eventually arrives and frees the couple.
Sherlock returns to the bank to tell them that he found the hole in their security and delivers some good news to one of the assistants and the bank.
“The Great Game” (Episode 3)
Mycroft asks Sherlock to investigate the suspicious death of a government employee, Andrew West, who was working on a top-secret missile project. Sherlock declines the case and passes it to Watson.
Sherlock is summoned to Scotland Yard by Lestrade. After opening a package, Sherlock is contacted by a criminal who forces Sherlock to solve a puzzle, or he will kill his victim. Sherlock deduces, due to pips on the call, that he will be required to solve five puzzles. That turns out to be the case.
Sherlock realizes the common thread. He also contacts Moriarty, offering to turn over the missile plans at an in-person meeting. When he arrives, he is greeted by Watson, who is wearing an explosive vest and has the telltale red laser light of a rifle pointed at his chest.
Moriarty finally shows and taunts Sherlock before turning to leave. Sherlock rescues Watson, but only briefly. Moriarty returns to kill them after all, but Sherlock throws a wrench in his plans.