An explosion at Parabolic Web Industries, a small web design firm, has killed two people, and Sherlock is investigating. The first thing he notices is the presence of an old pager, which he notes is an unlikely device to find at a web design firm using bleeding edge technology. He determines that it was part of the bomb and has Gregson check who called it. This leads to the first suspect, Rennie Jacobs, who claims that he called a sandwich shop and dialed incorrectly. Sherlock confirms this story when he inspects the bomb. He notes that a picture of Senator Barack Obama on the bomb's newspaper packing, and the design of the battery, mean the device was made in 2008. This means that Jacobs, who was in prison at the time, can't be guilty.
By the bomb's age, Sherlock deduces that the target wasn't Parabolic at all, but the company that previously used that location: Van Owen Strategic Communications, a PR firm that specializes in solving corporations' image crises. Sherlock questions the CEO Heather Van Owen, who admits that defending oil companies' reputations annoyed environmentalists, and states that she received threatening letters from an eco-terrorist. Sherlock reads the e-mails repeatedly until he notices a phrase used several times in the messages. He remembers that phrase was used on TV, and searches his recordings until he finds it. It was said by eco-terrorist Edgar Knowles. Knowles is arrested and interrogated by the police. Knowles states he is innocent, but when they tell him they found his fingerprints at a lumber mill bombing, he confesses to that but still insists he did not bomb the web firm.
At the Brownstone, Watson hears explosions from the roof. She finds Sherlock setting off tennis ball bombs. When asked, he states he is setting off bombs with the formula Knowles used for his bombs. Sherlock notes the smell is different from the web firm explosion and that eco-terrorists pride themselves on using natural ingredients.
Gregson complains that Sherlock brought him the bomber, only to declare him not guilty. Sherlock shows him a newspaper shred with the word "Novocaine" written on it and suggests the bomber was an employee framing Knowles. Sherlock goes to the firm and checks the files for a suspect. While searching, Van Owen notes him as a addict, of crossword puzzles. Sherlock recognizes this as flirting, and tells her that he is busy at the moment. Watson shows up, causing Van Owen to leave. Sherlock tells Watson he found a potential suspect named Pradeep Singh, the company's web designer who had received many promotions over a short period of time before getting into a fight with a boss, being fired, and disappearing.
Sherlock visits Singh's wife who believes that he is dead because he would never leave her. Sherlock asks if she has renovated the room, and when told no makes an excuse to get her out of the room. Sherlock inspects a wall in the room, and notes that it is moldy, feels cold, and the pictures have been moved slightly. Sherlock goes outside and tells Watson that Singh was killed and buried within the walls of the house. Sherlock has torn down the wall and sure enough, Singh's body was been decomposing there for years. Sherlock looks at a picture of Singh in his office and realizes from the vent's position that he was in fact the intended victim. Bell brings him a key from the body, which Sherlock recognizes as a safety deposit key. They go to the bank and find a VHS tape, and Sherlock notices the copy protection tab was removed and replaced by scotch tape- it has been recorded over.
Sherlock takes it home with him and views it on his VHS player. That is when he realizes it is a sex tape between Van Owen and Singh. He takes the video to the police and they bring in Van Owen to show her. Gregson tells her that with her humble background, she had to resort to prostitution to afford college. One of her clients, Singh, recorded the session and later used it as blackmail when he ended up working for her. Sherlock then shows her a shred of paper from the bomb with the word "Novocaine" in her handwriting, the solution to the crossword puzzle of the newspaper that the bomb was packed with.
This episode contains examples of:
- Accidental Murder: The bomb was designed to kill someone, but the person it was supposed to kill had moved from that position, and the thing was only detonated because a guy misdialed while trying to call a deli.
- Actor Allusion: Heather Van Owen is revealed to have put herself through business school by working as a High-Class Call Girl. She's played by Lisa Edelstein who had a recurring role in the first season of The West Wing as a woman putting herself through law school by working as a call girl.
- Blackmail: Pradeep Singh did this to his boss, Heather Van Owen, with a video of him sleeping with her as a prostitute. This leads to her killing him.
- Body in a Breadbox: While visiting the Singhs' house Sherlock notices that the pictures on the wall are hanging lower than in the photograph of the same room displayed on a shelf. He gets Joan to take Mrs. Singh outside so he can investigate and discovers that Pradeep Singh's body has been sealed up behind that patch of wall ever since he went "missing".
- Both Sides Have a Point: Sherlock is skeptical that the NYPD has pulled in the right man as their bombing suspect; Watson points out that deductive observation can only tell so much:Holmes: Bomb building is a dangerous venture. It requires patience, precision, attention to detail. Mr. Jacobs' wristwatch is nine minutes slow and his fly is three-quarters unzipped. That doesn't scream "detail oriented."
Watson: Ted Kaczynski looked like a hobo puked another hobo. He managed to hurt plenty of people.
Holmes: And point taken. - Chekhov's Gun: Heather Van Owen's love of crossword puzzles is what reveals why the word "Novocaine" was found in the bomb packing.
- Chekhov's Skill: Heather Van Owen, the head of a successful Manhattan public relations firm, says she wouldn't know how to seal a body into a wall; but Holmes points out that her father ran a contracting business.
- Constructive Body Disposal: Holmes asks Watson to find some pretext to take Mrs. Singh outside, while he confirms his theory that her husband's corpse was walled up inside their new home. Then he comes outside and asks Watson to find some pretext for her to stay outside for another few hours.
- Corrupt Corporate Executive:
- Pradeep Singh, a web designer for a corporate PR firm blackmails his boss when he realized he'd slept with her, and filmed it, while she was a prostitute.
- Said boss, Heather Van Owen, responds with two attempts to kill the employee; one is successful, one winds up killing two innocent people four years later.
- Eco-Terrorist: An eco-terrorist named Edgar Knowles becomes a suspect in a bombing when his distinctive pattern of speech is matched to some threatening emails. Holmes rules him out after realising the bomb used a chemical-based fuel, and Knowles only ever used natural fuels in his bombs.
- 555: The phone number dialed to a pager that detonates a bomb.
- Funny Background Event: Funny foreground event: Detective Bell has been questioning Edgar Knowles, the Earth Liberation Militia guy they think planted the four-year-old bomb, when without warning Holmes bursts in and begins lecturing them both about having successfully taken the guy's fingerprint. Bell sits in the foreground to Holmes's right, staring straight off into space with a look of "oh for @*$#'s sake" on his face, for fifteen whole seconds.
- I Was Young And I Needed The Money: Heather Van Owen put herself through business school by working as an escort. Holmes and Gregson agree that this is not a crime, but killing Pradeep Singh for threatening to expose her was.
- Not Me This Time: Edgar Knowles may have set off bombs in the past and threatened the PR firm, but it was someone else who planted the bomb.
- Poor Communication Kills: Zig-Zagged. Heather's bomb initially failed to explode because the pager she used as a detonator was too far away from a cell tower to receive a signal; by the time the manufacturer had built a tower close enough, her office and her intended victim had both moved to a completely different building, which means that when the bomb detonated, it killed the wrong people.
- Sherlock Scan: Inverted. Based on one of these, Holmes is skeptical of the police's obvious suspect, Mr. Jacobs, but Watson points out that a scan is subject to error like any other method of detection, and Holmes agrees she has a point.
- Technology Marches On: In-Universe. Holmes is able to deduce that the bomb was planted years before it actually went off, because its remote detonator is a pager instead of a cell phone. Moreover, the bomb didn't detonate when it was supposed to because the telecommunications company hadn't built any cell towers within range of the building, but after it did so, the bomb was inadvertently detonated by a wrong-number call from a cell phone.
- Unwitting Instigator of Doom: The bomb that kicks off the episode was accidentally triggered by a man trying to call a similar number to the one that activated the explosives.
