Here's a summary from the PBS site:
A spunky heroine braves assassins, opium dens, and worse to get to the bottom of her father's mysterious death in this adaptation of Philip Pullman's Victorian-era thriller The Ruby in the Smoke. Pullman's 1985 novel introduced teenage sleuth Sally Lockhart, a self-reliant orphan schooled by her late father in shooting, bookkeeping, and Hindustani.
The story opens in London in 1872. Sally's father, in the South China Seas investigating suspicious activity in his shipping business, has gone down with the schooner Lavinia. After receiving an anonymous letter postmarked 'Singapore' to beware of the 'Seven Blessings,' she asks one of her father's business associates about it; he clutches his heart and drops dead on the spot.
Mr. Lockhart's old friend Major Marchbanks gives Sally a journal that tells the story of the Ruby of Agrapur, which disappeared during the Great Indian Mutiny of 1857. Sally is about to read the passage describing her father's role in this event, when the journal is stolen out from under her! Thus begins a chase leading to dastardly deeds in high places and a shocking revelation about Sally's family history.
British actress and former pop idol Billie Piper (Doctor Who) stars as Sally, the young heroine thrust into the role of detective. Julie Walters (Harry Potter) stars as Sally's nemesis Mrs. Holland, a homicidal crone obsessed with finding the fabulous ruby and convinced that Sally holds the key to its whereabouts.
Also appearing are JJ Feild (To the Ends of the Earth) as photographer Frederick Garland who offers Sally a hideout and avidly joins her high-risk adventure; Matt Smith as Jim Taylor, an office boy fond of "penny dreadfuls" which have taught him a thing or two about crimesolving; and David Harewood (Blood Diamond) in a dual role as the Bedwell twins: one an opium-addicted sailor who was the last to see Sally's father alive, and the other a clergyman not above a fistfight if the cause is just.
Author Philip Pullman is best known for his fantasy trilogy, His Dark Materials, set in a series of alternate universes. By contrast, the Sally Lockhart mysteries are firmly set in the universe of 19th-century sensationalistic fiction. Pullman calls The Ruby in the Smoke an "old-fashioned Victorian blood-and thunder... with a genuine cliché of melodrama right at the heart of it, on purpose: the priceless cursed jewel."