Dr Hellen Oswold returns to Portsmouth after a humiliating interview at a London university. On the train home she meets a quiet, elegant stranger who seems to know more about her research — and her private life — than he should. She wakes the next morning with two small puncture wounds on her neck and no memory of the journey home.

Hellen begins experiencing unsettling symptoms. Sunlight hurts. Food tastes metallic. She blacks out for hours at a time, waking in unfamiliar places. Her dreams fill with a dark figure whispering in an unknown language. At first she hides the symptoms, terrified that colleagues already consider her unstable after a recent breakdown.

Crows begin gathering outside her flat, watching her windows. Students report seeing her wandering the corridors after hours, even when she is home in bed. A colleague discovers her in an empty lecture hall with blood on her hands and no understanding of how it got there.

Convinced she is being hunted, Hellen researches vampiric folklore, discovering unsettling parallels between ancient symptoms and her new behaviour. But the deeper she investigates, the more her sense of reality fractures. The elegant stranger reappears at moments she cannot logically explain. A local historian suggests that Portsmouth has its own buried myths — stories of sailors returned from long voyages carrying something dark in their veins.

Hellen’s friends grow frightened as her personality shifts. She becomes sharper, stronger, colder. She develops heightened senses and violent cravings she cannot control. Yet her blackouts also reveal moments of vulnerability: messages scrawled in notebooks she does not remember writing, warnings seemingly left by her own future self.

As her condition worsens, Hellen tracks the stranger across the city’s backstreets, docks and cemeteries. The final confrontation takes place in the ruins of an old naval storehouse, where truth and hallucination bleed together. Is the stranger a vampire, a projection of her trauma or a manifestation of her fractured mind? The film leaves just enough ambiguity for both readings to be true.

In the climax, Hellen must decide whether to embrace the transformation overtaking her body or fight it — even if resisting means destroying the part of herself that finally feels powerful. The ending leaves the audience questioning whether she has defeated a monster or become one.