London's Metropolitan Police is overwhelmed by a series of suicide cases, where all the deceased have died after ingesting lethal poisonous pills. Consequently, a desperate Detective Inspector Lestrade is forced to turn for help to Sherlock Holmes—the unofficial consulting detective. From the subtle clues in the case, Sherlock deduces that these incidents are not mere suicides, but the work of a serial killer pulling the strings. Meanwhile, John Watson, who is also looking for a flatmate, is introduced to Sherlock. The two hit it off instantly and immediately move into the flat at 221B Baker Street.
Through deduction and pursuit, the killer's true identity is finally revealed—a London cab driver. Sherlock confronts the murderer alone inside a deserted university building. There, the killer explains his method: he points a gun at his victims, forcing them to participate in a lethal game of Russian roulette with pills—two capsules, one harmless, one fatal. After the victim makes their choice, the killer picks the remaining capsule, and both swallow their pills simultaneously. However, has the killer, having won five times in a row, truly been favored by fate?
As the situation becomes critically dangerous, Watson, who has followed them, raises his gun and shoots the cab driver. Under Sherlock's intense questioning, the dying killer finally reveals the true mastermind behind the entire scheme—Moriarty.