During World War I, the vampire Armand Tesla stalks London, leaving a chilling trail as he preys on the living. His latest victim is brought to the clinic run by Lady Jane Ainsley and her colleague, Walter Saunders. The medical team suspects a severe case of anemia, but the bite marks and the uncanny symptoms reveal something far holier and older than ordinary illness. Tesla infiltrates the clinic, and after failing to finish his prior victim because she dies of shock, he fixes his sights on Saunders’ granddaughter, Nicki Saunders. Saunders himself becomes convinced that a vampire is at work, insisting to Lady Jane that the puncture marks on both patients’ necks point to a supernatural force rather than a mere medical anomaly.
Together, Saunders and Lady Jane trace the uncanny occurrences to a cemetery where they confront the crypt of the vampire. There, in a suspenseful moment, Tesla is found in his coffin and a mirrored test reveals no reflection, cementing the grim truth: Tesla is indeed a vampire. A menacing werewolf named Andréas serves Tesla, trying to thwart the investigators, but their evidence is strong enough to end the immediate threat: once the vampire is spike-killed, Andréas is cured of his curse and later becomes Lady Jane’s trusted aide.
Twenty-four years pass, and the world has changed, yet the mystery deepens. Scotland Yard’s Sir Frederick Fleet reads Saunders’ account and warns Lady Jane that if the staked body was alive, she could face murder charges. Lady Jane explains that the man they killed was Armand Tesla, a legendary eighteenth-century vampire expert; she also reveals to her son, John Ainsley, that Sir Frederick will likely find the body’s condition a key clue. The family resolves to keep the truth from Nicki Saunders to spare her from reliving the earlier trauma.
After a bombing raid, cemetery workers discover Tesla’s corpse exposed in the ground with the spike still in his chest. They mistake the spike for bomb shrapnel, remove it, and reinter the body, allowing the vampire to reclaim power over Andréas and vow revenge on Lady Jane. Tesla even attempts to impersonate a scientist named Hugo Bruckner, with the real Bruckner’s body rifled from a drawer, complicating the investigation as Sir Frederick’s inquiry stalls for lack of clear evidence.
To celebrate the engagement of John and Nicki, Lady Jane hosts a lively party, balking at the idea of handing Saunders’ manuscript to Nicki and locking it away in a drawer. Tesla appears in disguise as Bruckner, charming everyone except Sir Frederick, and the manuscript is stolen and left in Nicki’s room. She awakens to find herself reading the manuscript and soon falls under Tesla’s thrall, later discovered unconscious with puncture marks at her neck.
Sir Frederick grows increasingly suspicious. He questions gravediggers about the spiked body and learns that the thread of vampiric lore might be closer to truth than he imagined. The search team tails Andréas, who transforms before their eyes, and they recover a bundle of evidence including wolf hair in a laboratory analysis. Realizing the danger Tesla poses, Sir Frederick closes in and orders plainclothes men to shadow the vampire and his werewolf ally. Tesla's threats escalate, vowing to turn Nicki and John, while Lady Jane stands firm, brandishing a cross and reminding everyone that “the power of goodness” remains a force against evil. Tesla finally vanishes, leaving the door open for a final confrontation.
The climactic sequence brings Nicki, Tesla, and Andréas to a cemetery amid an air raid. Andréas is badly wounded trying to shield Nicki, and Sir Frederick shoots him to prevent further catastrophe. They seek shelter in the burned church ruins, where Jane’s memory and a crucifix seem to weaken Tesla's influence. In a tense struggle, Andréas, guided by Jane’s words, rebukes the vampire and seeks to destroy him, but a bomb tears through the church and knocks them unconscious. When the dust settles, Andréas drags Tesla into the daylight, and the sun’s rays destroy the vampire down to bone. Tragically, Andréas dies from his earlier gun wound, but Nicki survives and reveals to Sir Frederick and Lady Jane that Andréas saved her life.
With the immediate threat gone, Sir Frederick remains cautious. He resists embracing the vampire story without undeniable physical proof, turning to his deputies for their belief. Their affirmative response lingers in the air as he faces the stubborn, unsettling truth about what happened and what might still be possible. The final moment leaves a haunting question hanging in the room:
> And do you, people?
