A young man, Ron, walks down to the docks at night and notices a young woman, Hyacinth, sitting on the deck of a barge. He offers to cross over and talk to her, but Hyacinth refuses and says that she knows his name and that he works at the pier.
The next day, Ron is working at the store with his partner, Jake. Ron's girlfriend Phyllis comes by to see him, but he says that he has things to do that night and can't go out with her. After Ron leaves, Jake says that Ron goes to bed every night at 7, sleeps until midnight, and then disappears until dawn. Later, Phyllis goes to see Ron as he's sleeping, and he admits that he's fallen in love with someone else.
The next night, Ron begs Hyacinth to let him come over, but she says that he must stay away for his own sake. Ron wonders if she's lonely and Hyacinth says that she has her father, but he's the only one who can leave the barge. She notes that the water in the canal is dropping lower every day because they're draining it to make a new marina. When Ron asks to meet her father, Hyacinth says that she sleeps during the day and her father sleeps during the night. She tells Ron that she has to go because it's almost dawn and retreats inside. As Ron leaves, he's unaware that Phyllis is watching from hiding. She notices Hyacinth's father come out, puts down the plank, and crosses to the dock and walks away.
Phyllis crosses over to the barge and enters the deck below. She watches Hyacinth prepare to enter a coffin. Hyacinth sees her and advances on her, and then grabs her throat. Phyllis manages to retreat to the deck and the sun's rays hit Hyacinth, driving her back so Phyllis can escape. Phyllis goes to tell Jake what she saw, and starts to explain that she looked up books in the library. Ron wakes up and leaves, and Phyllis reads Jake stories of vampires and how to kill them.
Ron goes to see Hyacinth, who accuses Ron of bringing a girl with him. He has no idea what she's talking about and then asks about her father. Hyacinth explains that she can't cross running water, and her father keeps her as a prisoner. However, she tells Ron that once the lagoon is drained, she'll be waiting for him one night and they'll be together. Hyacinth goes back inside and Ron starts to leave, and Jake comes up behind him. He suggests the share the girl and Ron shoves him to the deck and walks away.
Come morning, Hyacinth's father closes the coffin on his daughter and leaves.
The next day, Jake and Ron discuss Hyacinth as they serve their customers. Jake wonders why Ron is so attracted to her, given Phyllis' warnings, but Ron says that he wants to learn the truth. Later, Ron wakes up and Phyllis is waiting for him. He accuses her of spying on him, but Phyllis explains that Hyacinth and her father used to live in Congers, and three mutilated corpses turned up dead. Ron refuses to listen to her and Jake dismisses Phyllis' concerns. When Ron goes back to the barge, he discovers that her father has left the plank in position and the canal water gone. He looks through the nearby crates, wondering if she has left the barge, and Hyacinth emerges from a nearby cabin. She asks Ron to hold him but he hesitates, and she wonders if others have been talking about her and the corpses in Congers. She starts to kiss him, saying that she's been starving for the last year, but Ron stops her and says that he loves her. Hyacinth says that she loves him as well and lets him go, but tells him to never come back. Once Ron leaves, Jake comes up behind her.
Ron returns home and finds Phyllis waiting for him with dinner. They kiss and he offers a toast to a normal life, and is interrupted when the phone rings. It's the Coast Guard, reporting that Jake has been found dead in an accident. Ron drives to the beach and confirms that Jake is dead, and the coastguardsman says that they spotted him on the beach and found him with his throat cut open. Ron says that he'll follow them after making one stop.
Ron goes to the barge and goes below-deck, where Hyacinth is waiting. He tells her what happened to Jake and she says that she didn't mean to go so far. Hyacinth insists that Jake was deliriously happy right to the end, and explains that her victims all return. She reaches out for Ron and asks him to hold her, but he refuses. Ron explains that he came there to destroy her by taking her out to the beach to wait for the sunset. Hyacinth says that he doesn't have to, and his father has an oak stake in a nearby chest. Ron gets it and prepares to thrust it into her, and Hyacinth asks him to declare his love for her as he does. Hyacinth says that she loves him and Ron drops the stake, overwhelmed by his feelings. He enters her embrace and she bares her neck, but her father enters and plunges the stake into her heart. In seconds she decays into a skeleton. Shocked, Ron staggers away.