In late 19th century London, the eccentric and sharp-witted detective Sherlock Holmes and his steadfast companion Dr. John Watson prevent a ritual murder by apprehending the sinister Lord Blackwood. Blackwood, a follower of dark mystical practices, warns of more deaths and chaos to come, as he is sentenced to hang. Following Blackwood's execution, Holmes busies himself with other cases and peculiar experiments, displaying his trademark blend of brilliant deduction and erratic behavior. Meanwhile, Watson, seeking a more conventional lifestyle, plans to marry his fiancée Mary Morstan. However, his plans become complicated as he is inevitably drawn back into Holmes's world of intrigue.

The plot thickens when Blackwood seemingly rises from the grave, prompting rumors of his black magic bringing him back from the dead to fulfill his ominous promise. London is gripped with fear, and the authorities desperately seek Holmes's expertise. During the investigation, Irene Adler, a former adversary and love interest of Holmes, reappears with her own mysterious agenda, leading Holmes on a chase across London. He deduces that Blackwood's resurrection and subsequent murders are part of a larger and more diabolical scheme, with potentially catastrophic consequences. Holmes and Watson navigate a world filled with secret societies, occult references, and scientific advancements, piecing together Blackwood's plan that could change the course of history. They must use all their wits and bravery to thwart Blackwood's plot as they delve deeper into a world where logic and the supernatural collide.