In London, Vivian Vedder visits coffin makers Mock and Son to verify the completion of a coffin for her mother's body, which she is transporting to Scotland that evening on the Scotch Express. That night at Victoria Station, detective Sherlock Holmes meets Ronald Carstairs, who, with his mother, Lady Margaret, has hired Holmes to protect their most valued possession, the infamous Star of Rhodesia diamond, during their return journey to Edinburgh on the Express. Ronald admits that Holmes is correct in assuming an attempt was made to steal the bauble during his and Lady Margaret's visit to London. While waiting for his partner, Dr. John Watson, Holmes observes the arrival of Scotland Yard's Inspector Lestrade, who claims to be going north for a fishing vacation. Holmes quickly deduces that Lestrade is actually making the trip as official protection for the renowned gem. As the Express pulls out of the station, the late-arriving Watson just manages to board the train with a companion, Major Duncan-Bleek, whom the doctor introduces to Holmes as a member of his club. After the train is on its way, Holmes and Watson examine the 423- carat stone with Ronald and Lady Margaret. Later in the dining car, as Lady Margaret waits for Ronald, Holmes finds an anonymous card on his plate with a written warning. Meanwhile, Lestrade, in his compartment next to the Carstairs', hears a strange noise and upon investigating with Holmes, discovers Ronald's dead body and the empty jewelbox. Watson finds no marks on the body except for a small scratch, and Holmes suspects that Ronald was poisoned. Lestrade and Holmes begin questioning the passengers and note that Lady Margaret is more concerned over the theft of her stone than the murder of her son. Upon discovering that Vivian is in mourning, Lestrade and Holmes demure from interrogating her and turn to questioning Professor Kilbane, a mathematics instructor, who resents their inquiries. When Lady Margaret is discovered secretly attempting to enter her restricted compartment, Watson and Duncan-Bleek suspect that she has arranged to have the jewel stolen to collect the insurance. Holmes, however, postulates that the crime reflects the interests of the cunning Colonel Sebastian Moran, a cohort of Moriarty, Holmes's criminal archrival. While Lestrade questions Lady Margaret further, Holmes begins following Kilbane toward the luggage compartment. The professor abruptly disappears and when Holmes comes across an open passenger door, he is suddenly pushed through the door which is then locked behind him. Holmes narrowly escapes being hit by an oncoming express, then breaks the glass in the door to get safely back inside. Holmes and Watson proceed to the luggage compartment, where they inspect the Vedder coffin, which is far too large for the lightweight body inside. After more exploration, Holmes locates a secret compartment under the body, large enough for a small adult, which leads the detective to the conclusion that two people are involved in the crime. When Holmes and Lestrade question Vivian, she admits that a man paid her to transport the coffin. Watson and Duncan-Bleek join the group and Holmes astounds them all by revealing the Star has not been stolen, but was in his possesion the entire time as he used the opportunity of examining it earlier to switch the genuine stone with an imitation. Lestrade quickly takes possession of the authentic jewel. Later, when Holmes and Watson return to the luggage compartment, they find the porter has been murdered by a tiny poisonous dart made out of a dissolving substance. Meanwhile, unknown to the others, Duncan-Bleek is joined in his compartment by Sands, a rough street criminal who came aboard via the coffin's hidden compartment and is the major's cohort in the crime. Duncan-Bleek informs Sands their stone is a fake and sends him to get the gem from Lestrade, but after Sands knocks out the investigator, Duncan-Bleek kills Sand using an airgun with the poisonous darts and takes the Star. Holmes and Lestrade discover Sands's body just as the train makes an unexpected stop in a small village to pick up several policemen led by Inspector McDonald of the Scottish police, who have come aboard to take over the investigation. Lestrade, still groggy from being beaten, asks Holmes and Watson to deal with the inspector. To Watson's amazement, Holmes informs McDonald that Duncan-Bleek is really Sebastian Moran, but when McDonald arrests Duncan-Bleek and the police lead him away, the lights on the train are extinguished and a scuffle ensues. Holmes recaptures Duncan-Bleek by throwing a coat over him in the darkness, and the officers hustle away their prisoner. As the train departs, Holmes reveals that McDonald and his men were imposters in league with Duncan-Bleek, whom Holmes had secretly handcuffed and hidden under the table during the scuffle. McDonald's prisoner is actually Lestrade, whose face was obscured by the coat. With the capture of the thieves, Holmes then admits that he has pocketed the valuable Star of Rhodesia.