In this episode, Sherlock finally meets the mastermind behind the cases from the first two episodes.
Following an explosion, Sherlock receives a pink mobile phone from Lestrade. After five beeps, a picture arrives via text message. Sherlock goes to the location shown in the picture and finds a pair of shoes. Simultaneously, a distressed woman calls. If the puzzle isn't solved within 12 hours, she will be blown up. After successfully solving the case, the phone emits four beeps. Another victim has been fitted with a bomb, and this time there are only eight hours to solve the mystery of the car splattered with blood. Subsequently, each time a case is solved, a new puzzle and a new kidnapped innocent person appear.
Meanwhile, Mycroft also commissions Sherlock to help find the missing Bruce-Partington missile plans, but Sherlock completely ignores the request. The unable-to-refuse John is left to work on it himself. After successfully solving the case of the famous TV presenter's death and the case of the forged painting, no new bomb victims appear.
Sherlock finally decides to help John with the Bruce-Partington affair. After successfully locating the missing USB stick, Sherlock conceals his decision to meet the culprit alone. When Sherlock arrives at the appointed place, the person who appears is none other than John, who was supposedly on his way to Sarah's house! It turns out that John has also been taken hostage as a bomb victim, and the real culprit is Jim Moriarty—the man who was introduced as Molly's boyfriend in the first episode.