The film begins with a young woman climbing into the bath and slashing her wrists. Her husband, Stephen (Richard Harrington) comes home and finds her dead. Some time later, he is in hospital visiting his mother, who is dying, when he bumps into his boss, Eisner (Mark Lewis Jones). He asks his boss if he can come back to work, and his boss agrees, giving him some medication.
The next day, in work, he is given a patient, a teenage girl called Nina (Jaime Winstone), who has slashed her wrists. Nina tells him that she didn't mean to do it, she just fell. Her mother Liz (Louise Delamere) enters and tells him that she doesn't want Nina seeing a psychiatrist but he argues that it is in her best interests. As he finds out more about her, she tells him that she sucked the blood so people wouldn't find out what she had done. He tells her about his mother, who is dying. When he leaves, Nina sees a boy having a blood transfusion and asks if she can drink it, but is interrupted by the nurse.
Instead, she goes to find Stephen's mother and drinks her blood instead. The hospital call Stephen and tell him that his mother has had an accident, and he arrives there and argues with the nurses. Later on, he is outside, when Liz approaches him and she tells him some more about Nina. Nina watches from her room.
Some time later, Stephen is at his mother's funeral. He sees Nina in the distance and goes to talk to her. She tells him she felt bad for him, and wanted to see that he was OK. He takes her home, and talks to her. Stephen's boss warns him about getting involved with patients but he tells him there is nothing wrong with what he is doing.
One night, his plumbing leaks so he ends up spending the night with Liz, when she calls him to tell him that Nina has assaulted her. She tells him about Nina's father being found with a needle in his arm, when Nina was 6. They end up sleeping together, and go out the next day. Nina brings another patient of his, Lucy over, and sucks her blood. Later, Lucy tells him the problem.
He interviews Nina, and she is very rude to him, telling him she knows he is fucking her mother. A member of staff overhears this and keeps the tapes. He goes to tell Liz about Nina, and they kiss, but Nina sees this and runs away to the train station, where she bites a man's lip for some blood.
She arrives at Stephen's flat where he washes her hair (using heated water from the kettle, because he disconnected the water supply) and puts her to bed. She later stands over him with a needle, but we do not see her inject it. The next day, at work, he is seen bleeding, and injects himself with something, despite telling Nina previously he was scared of needles. His boss catches him and tells him that he is suspending him over his relationship with Liz.
He goes back home and drowns himself in the bath, only for Nina to rescue him. She ties him up and sticks needles in him, and tells him about a tribe who survive by drinking the blood of their cattle. An estate agent arrives and sees him semi naked on the floor, and berates him for this, only for Nina to attack him from behind. Liz enters and the screen fades to black.
Later, Stephen's boss is with Liz, telling her that he killed the estate agent and disappeared. Liz then goes to the basement and gives Stephen some water to keep his strength up. Nina comes in and sucks his blood through a straw and the film finishes.