Ron, a local merchant in a small seaside town, is walking past the canal at night when he sees a young woman in a white dress sitting on a barge. He approaches the dock and she introduces herself as Hyacinth. She refuses to come across to meet him, but says that she’s seen him walk by and knows his name. Ron contents himself with talking to her across the canal water that separates them.
The next day, Ron is at his shop working with his partner, Jake. Ron’s girlfriend Phyllis comes in to see what he’s doing that night, but he says that he has things to do and can’t go on their date. Ron goes and Jake tells Phyllis that his partner has gone to bed every night and sleeps until midnight, and then leaves until midnight. That night, Phyllis goes to see Ron as he is sleeping and wakes him up. Ron tells her that he’s fallen in love with someone else.
Ron continues to meet with Hyacinth and begs her to come to him, or to let him come to her. She insists that Ron has to stay away for his own sake. When Ron asks how she stands to be alone, Hyacinth explains that her father lives on the barge, sleeps during the night, and works during the day. Ron suggests that he cross the water, and Hyacinth says that it is growing shallower every day as they drain it for the new marina. Ron asks to see her during the day and meet her father, and Hyacinth insists that she can’t let Ron do that. As dawn approaches, Hyacinth says that she can hear her father waking up and goes below. As Ron leaves, Phyllis watches from the shadows. She sees Hyacinth’s father come up on deck, put down the plank, and leave for work.
Once Hyacinth’s father has left, Phyllis crosses over and enters the barge. She finds Hyacinth below, preparing to enter a coffin. Hyacinth advances on Phyllis and grabs her by the throat. They struggle up the stairs and Phyllis manages to open the door, and the rays of sunlight strike Hyacinth. Shocked and in pain, Hyacinth falls back and Phyllis runs away.
The next night, Phyllis arrives to tell Jake what happened, but he doesn’t believe that Hyacinth is a vampire. As she tells him that she’s been reading books, Ron wakes up and leaves, ignoring Phyllis’ suggestion that they do something together. She then tells Jake the stories of vampires who rest in coffins during the day, and feed on the blood of their victims. The only way to stop them is to restrict them with running water, or to kill them using sunlight a wooden stake.
Ron goes to Hyacinth, who tells him that Phyllis came onto the barge. He has no idea what she’s talking about, and asks about her father. Hyacinth tells him that her father keeps her as a prisoner and that she can’t cross running water, but can leave once they drain the canal. Once that happens, the two of them will be together. As the sun starts to rise, Hyacinth goes back inside and Ron turns to leave. Jake is there, curious, and suggests that if Ron doesn’t want Hyacinth then he’ll take her. Ron pushes him away and leaves. Inside, Hyacinth’s father smiles down at his daughter inside her coffin and then closes the lid.
The next day, Jake and Ron are serving their customers and Jake wonders why Ron is so obsessed with Hyacinth. Ron insists that he doesn’t know the truth, but he’s drawn to Hyacinth and wants to learn what she’s hiding. That night, Phyllis comes to see Ron. He accuses her of spying on him, but Phyllis insists that Hyacinth is dangerous. She tells her boyfriend that she has checked the papers and learned that a year ago there were three mysterious deaths in Congers, the last town where Hyacinth lived with her father. Jake, listening in, insists that none of it is real and dismisses her concerns, and Ron agrees. He goes to see Hyacinth, and discovers that the canal water is drained and she has come ashore. Hyacinth goes to embrace him but he hesitates and she asks if he’s heard about the deaths She asks Ron to hold him but he hesitates, and then says that he loves her. Hyacinth tells him that he loves her and then tells him to go before it’s too late, and to never come back. Ron does as she asks, but then Jake comes up behind her.
When Ron returns home, he discovers that Phyllis is waiting for him and has prepared dinner. The boy is free of Hyacinth’s spell and offers a toast to a normal life, but they’re interrupted when the Coast Guard call to have Ron identify a body. He goes to the beach and confirms that the body they’ve found is Jake’s. The coastguardsman explains that they found him on the beach, his throat cut. Ron says that he’ll follow them into town but has one stop to make.
Ron goes to the barge and finds Hyacinth below, sated on blood. She admits that she didn’t intend to kill Jake, but it went too far and Jake died happy. She asks Ron to hold her, but he hesitates. Finally he says that he must take her out to the beach and let her die in the sunlight. Hyacinth asks him to spare her the pain by using the wooden stake that her father has stored in a chest and impale her through the heart. Ron picks it up and prepares to deliver the killing blow, and Hyacinth asks if he loves her. She says that she loves him and Ron collapses into her arms, sobbing. Overwhelmed, Hyacinth bares her fangs and prepares to bite his neck, but sunlight suddenly shines on her face as her father enters the hold. He takes the stakes and thrusts it through her heart, reducing her to a skeleton in a matter of seconds. The two men look down at her in sorrow and regret, and Ron leaves.