Sherlock has been hired by the father of a rape victim. He has been receiving notices, demanding money or the video of her rape gets released to the public. The notices warn that if he, the blackmailer, is incarcerated of killed, his partner will release the videos. Sherlock is able to find out where he lives by the bank account he provided. Sherlock breaks into the apartment of the blackmailer, Charles Augustus Milverton, to destroy the material, but he returns ahead of schedule. Another man breaks into the apartment, guns him down, and takes the laptop containing the video, but Sherlock does not see the man's face because of a ski mask.
Sherlock realizes that if Milverton's death got out, the reputations of the victims would be ruined. He shows Gregson one of the blackmail videos to prove the stakes of keeping his death quiet. The first person they seek out is the rapist, Brent Garvey, who is in the prison infirmary. Garvey states that he was visited by Milverton, but he is not affiliated with him and is another blackmail victim.
Alfredo reports that he saw a man with a suit and cowboy boots leave Milverton's apartment. Sherlock deduces that the the man was Duke Landers, an attorney known as "The Sheriff" who had gotten Milverton out of a DWI charge. Sherlock decides to visit him in his law office. At Landers' office, Sherlock asks him what he knows about Milverton. Landers refuses to disclose any information, citing lawyer-client confidence. However Sherlock notices the diplomas on Lander's wall, and inspects them closely. Sherlock determines that the signatures are forgeries, and threatens to call the universities. Landers admits that Milverton had him report any useful information to him.
Back at the police station, a man named Anthony Pistone was arrested when he was found dumping the body of Milverton. Pistone claims he paid Milverton in cash to protect his stepdaughter, tracked him to his home, and killed him. He then smashed up the laptop, threw it in the dumpster, and when he was caught stomped Milverton's face in a fit of rage.
Sherlock looks at Milverton's budget sheets and notices payments to "Henry VIII", and realizes that this is a code name for an obese man. He finds out about a man named Abraham Zellner who filed an inconvenience lawsuit against a movie theater for his weight, but took the lowest offer provided. Sherlock tells Gregson to look up this man, but Gregson does not find any records. Sherlock looks up similar lawsuits and notes that there is a pattern in the initials of the aliases: A.Z., B.Y., C.X., etc. When he discovers the name Stuart Bloom that does not fit that pattern, he deduces it must be his real name. He goes to the residence of Stuart Bloom, and finds the floors covered in cat litter. In the bathtub he finds Bloom's body with a footprint on his chest, and deduces that the killer was Milverton by the size of the footprint and the brand of the cat litter, which was used to cover up the smell of decomposition.
Sherlock realizes that the blackmail notices have been going out even though both Milverton and Bloom have died. He goes back to the station and confronts Pistone, who is about to be released. He tells Pistone that the real reason he stomped Milverton's face was that he was covering up a scar on his face that was shaped like his ring and would prove they had earlier contact. When Pistone protests that many people in the city have the same ring, Gregson points out that the laptop has found in his brother's desk, not smashed like he said, and his brother confessed sending out the notice. Pistone is then charged with blackmail and murder.
Tropes:
- Ambulance Chaser: Richard "Duke" Landers, a.k.a. "The Sheriff", the lawyer who worked with Milverton, passing on any juicy information that his clients "happened" to share with him. Based on Holmes's examination of his law school diploma and court certificates, it's doubtful whether he's even a real attorney.
- Asshole Victim:
- Serial Rapist Brent Garvey is assaulted in prisonnote , as well as blackmailed by Milverton.
- Stuart Bloom being killed by Milverton, given the two were involved in blackmail together.
- Bad Boss: Charles Augustus Milverton killed his partner Stuart Bloom for demanding a raise.
- Blackmail: Milverton extorted the family of rape victims by threatening to release the videos to the public.
- Death by Materialism: Stuart Bloom demands more money from Milverton, who acquired a second partner and was splitting the money between them, but Milverton instead killed Bloom.
- Demoted to Extra: While Milverton has more posthumous impact, he never shows up to visit Holmes and Watson and gets far less personality.
- Disposing of a Body: Averted. Milverton drowned Stuart Bloom in his bathtub, but because his body was too heavy to move, he spread a week's worth of cat litter around the home to obscure the smell of the body putrefying.
- Driven to Suicide: Ken Whitman admits that his daughter Eva "tried to hurt herself" after her Date Rape by Garvey. She didn't go through with killing herself, but he is terrified that she might try again if the blackmailer publishes the sordid details on the Internet.
- Failsafe Failure: Milverton thought he was safe as long as he had an accomplice who was ready to release his blackmail materials if anything happened to him. Unfortunately, the man he recruited as his new "failsafe" killed him and took over the business.
- Fat Bastard:
- Stuart Bloom, the blackmailing partner of Charles Augustus Milverton, Sherlock describes him as being able to "pull small moons out of orbit". He also makes a number of weight related lawsuits under aliases, such as suing cinemas or airlines because their seats aren't big enough for him, and accepts lowball settlements due to the alias not being able to hold up under scrutiny.
- Duke Landers is described as fat by Llamosa, and involved with the blackmail scheme, as well as Bloom's Frivolous Lawsuits.
- Hoist by His Own Petard: Milverton is killed by Anthony Pistone, who claims to be one of his blackmail victims. The actual reason for the murder still qualifies, as Milverton was willing to trust someone willing to sell out his family.
- Mythology Gag: "Dead Man's Switch" is basically a rewrite of "The Adventure of Charles Augustus Milverton" with the blackmailer dying in the first ten minutes and Holmes needing to track down the blackmail material before the blackmailer's partner can release it to the public rather than Holmes being able to destroy it all immediately after the blackmailer's death as in the original story.
- Never One Murder: When Milverton's partner is found, he is dead from being drowned in his bathtub. Holmes determines that Milverton is responsible, given the shoe print is too small to be Pistone's.
- No Honor Among Thieves:
- Milverton kills Stuart Bloom for demanding a raise.
- Pistone kills Milverton to take over the business, after finding out Bloom was killed.
- Anthony Pistone's brother is the one to reveal Anthony's real reason for killing Milverton.
- Lampshaded by Holmes and Alfredo: when Ken Whitman receives a fresh blackmail demand from "Charles Milverton" after Milverton has been killed, Holmes is actually relieved - instead of fulfilling his role as Milverton's "failsafe", the accomplice is taking over the business, which means Holmes and Watson still have time to track him down before any compromising materials are published.
- Papa Wolf: Alfredo's sponsor, Ken Whitman, paid the blackmailer's demands, and is desperate for Holmes and Watson to track him down, because he's afraid of his daughter Eva's reaction if Garvey's "rape tape" is published online. When asking Gregson to keep Milverton's murder a secret until the accomplice is found, Holmes asks the Captain what he would do if it was his daughter whose reputation and sanity were at stake.
- Pariah Prisoner: Visiting rapist Brent Garvey in the prison hospital wing, Sherlock cheerfully says "it's nice to know it's not just a rumor" that criminals who victimize children are not well-liked by their fellow inmates. When Garvey proves resistant to questioning, Sherlock threatens to share one of his "rape tapes" with the other inmates, who might get angry enough to "visit" him in his sickbed, while he's asleep.
- "The Reason You Suck" Speech: Alfredo gives one to Sherlock, when he is not willing to accept his sober anniversary chip, telling him the point of the chip is to tell other recovering addicts that it is possible to get clean and not everything is about himself.
- Siblings in Crime: Anthony Pistone had his brother help him take over the blackmail business.
- Sympathetic Murderer: Milverton's killer, Anthony Pistone, due to being a victim of blackmail. Subverted when we learn Pistone was in on Milverton's blackmailing and killed him to take over the business.
- Themed Aliases: Stuart Bloom uses a series of aliases when pursuing his nuisance lawsuits. The first one is Abraham Zellner. Holmes realizes that there is a pattern in the initials of the aliases: A.Z., B.Y., C.X., etc. When he discovers the one name that does not fit that pattern amid a series of weight-related lawsuits, he deduces it must be Bloom's real name.
- Who Murdered the Asshole: Milverton was a serial blackmailer currently threatening a rape victim's father with releasing a video of the rape on the Internet. In fact the investigation of his death is more about finding the titular "Dead Man's Switch" to prevent the video from being uploaded.
- Wicked Stepfather: Anthony Pistone claims to have murdered Milverton in defense of his daughter, but he actually was taking over the business. Gregson informs him that he discovered through the neighbors that Pistone's daughter was actually his stepdaughter and they had a poor relationship.
