At the National Historical Artifacts Museum, Soo Lin Yao [Gemma Chan], a skilled expert in Chinese pottery, finishes her day. After her co-worker Andy Galbraith [Al Weaver] asks her out and is politely refused, she warns that he wouldn’t like her if he truly knew her, and she urges him to stop asking. That night, as Soo Lin works late, she investigates a suspicious flutter of movement and discovers a statue shrouded in a canvas. What she witnesses is harrowing enough to spook her into quitting on the spot and vanishing without a trace.
Meanwhile, in a very different London, Sherlock Holmes [Benedict Cumberbatch] and his reluctant companion, Dr. John Watson [Martin Freeman], are drawn into a case of financial mystery and murder. John is strapped for cash and struggles to pay for groceries, while Sherlock, sensing a larger game, leads them to a powerful bank as if it were a living organism. Inside, they meet Sebastian Wilkes [Bertie Carvel], an old university acquaintance who hires Sherlock for a hefty fee. The bank has just endured a bizarre break-in: nothing stolen, yet a pair of cryptic symbols spray-painted on a portrait of the chairman. The wall and the painting both bear these symbols, and CCTV places the vandal inside the building during a precise sixty-second window between 11:33 PM and 11:34 PM.
Sherlock quickly narrows the message’s target to Edward Van Coon [Daniel Percival], an employee stationed at the Hong Kong desk who has not returned after a midnight shift. The coded message seems intended for someone who works late and travels China, and Sherlock suspects something is terribly amiss. A breakthrough comes when he breaks into Van Coon’s locked apartment and finds his dead body. The police, led by Detective Inspector Dimmock [not linked], begin to suspect suicide, but Sherlock insists on murder. A black paper lotus is found in Van Coon’s mouth, and the body shows a gunshot wound to the right temple—yet Van Coon was left-handed.
Shortly after, Brian Lukis [Howard Coggins] is found dead in his own locked flat. Sherlock and John press on, tracing the pattern of the two victims’ deaths. The investigators discover a black lotus left on the floor of Lukis’s apartment, the same symbol Sherlock pulled from Van Coon’s mouth. Clues lead them to a library book checked out by Lukis and to a railway yard wall where graffiti reappears. Andy’s path remains tangled with Soo Lin’s disappearance; the museum director tells him Soo Lin has resigned, and he leaves a note at her Chinatown door.
John finds employment at a local medical practice where Sarah Sawyer [Zoe Telford] works, while Sherlock and John deepen their connection to Soo Lin’s disappearance. The two victims had recently delivered something to a London shop named the “Lucky Cat,” and this clue eventually points Sherlock toward a cipher that ties to ancient Chinese Suzhou numerals. The symbols on the bank wall translate to the numbers 15 and 1, and the mystery deepens as Sherlock consults the museum’s records. He visits Soo Lin Yao’s apartment and unexpectedly encounters an attacker who leaves a Black Lotus flower in his pocket.
The trail leads back to the Antiquities Museum, where a familiar face among the graffiti points them toward a hidden code. With the help of a graffiti artist named Raz [Jack Bence], Sherlock and John uncover more masked symbols at a railway yard and eventually relocate Soo Lin Yao to a hidden corner of the museum. Soo Lin reveals that the cipher is the work of a criminal faction called the Black Lotus Tong, a group she once belonged to. The Tong offers a comfortable life to orphans, and its leader, Shan, has an assassin who turns out to be Soo Lin’s own brother. Before the code can be fully deciphered, the assassin strikes again, killing Soo Lin.
Holmes pieces together that Van Coon and Lukis were not simply smugglers but couriers for the Tong, transporting valuable antiquities from China to London. Their cover stories—Van Coon as a traveling businessman, Lukis as a journalist—masked the truth: they were smuggling, and one of them had stolen something valuable. The message encoded with a book cipher becomes the key, as Sherlock and John spend the night pouring through the Victims’ books to crack it. John’s first day at the clinic becomes chaotic when he naps at the desk, but Sara covers for him, and the trio agrees to attend a traveling Chinese circus to gather more clues.
During the circus interlude, Shan orchestrates the assassin’s entry into England under the guise of a performer. Sherlock uses the downtime to investigate backstage and narrowly avoids a trap, but with Sara and John by his side, the trio escapes unharmed. As Sherlock deciphers the book cipher, John and Sarah are abducted, and Sarah is strapped to a deadly crossbow that is ready to fire. The abductors force John to reveal Sherlock’s whereabouts, but Sherlock cracks the code in time, reads the message from a London map book, and locates the criminals’ hideout.
Sherlock rescues John and Sarah, and the missing treasure is revealed to be far more personal than expected: a jade hairpin once belonging to Amanda [Olivia Poulet], Van Coon’s secretary and mistress, a prize she had received from Van Coon himself. The hunt for Shan continues, but the Tong’s leader narrowly escapes capture when a sniper sent by the mysterious overseer known only as “M” shoots her, ending Shan’s life.
In the end, the misdirections unravel and the true story emerges: Van Coon and Lukis were smuggling treasures tied to the Tong, with the jade hairpin representing the prize they sought, and Amanda’s possession of the hairpin confirms the Tong’s far-reaching entanglements. The case closes with Holmes and Watson reflecting on the delicate balance between wealth, loyalty, and danger, and the shadowy power of a group that operates in the open and in the dark.
