Eight-year-old Seth Dove Jeremy Cooper lives in an isolated Idaho prairie community in the 1950s. The film opens with Seth and his friends, Eben Codie Lucas Wilbee and Kim Evan Hall, playing with a frog they found in the fields. The boys inflate the frog by threading a reed up its rear and leave it by the side of the road. When a local English widow, Dolphin Blue Lindsay Duncan stops to inspect the scene, Seth fires a slingshot at the inflated creature, causing it to explode over Dolphin.
Seth retreats to the small gas station where he lives with his overworked, harsh, longing mother Ruth Dove Sheila Moore and shy, closeted, detached father Luke Dove Duncan Fraser. Seth's older brother, Cameron Dove Viggo Mortensen, is away on military service in the Pacific, which Ruth often laments. A mysterious group of young men driving a black Cadillac stop by the station, promising to return, and then drive off again.
Seth is sent to Dolphin's house to apologize for the frog prank. Dolphin is haunted by the memory of her dead husband, who hanged himself for unknown reasons a week after their wedding. Surrounded by artefacts from her husband’s whaling past, Seth takes some of her self-pitying remarks literally. After learning about vampires from his father, Seth begins to wonder if Dolphin could be one.
> "two hundred years old"
After Eben goes missing, Seth and Kim go to Dolphin's house to investigate, suspecting her of involvement. They raid her bedroom and wreck her belongings, then hide to listen as she descends, moaning and touching herself. They are discovered, scream, and run. Seth spots the same black Cadillac outside, and later discovers Eben’s dead body floating in the water cistern. Local authorities quickly point to Luke, given his earlier inappropriate encounter with a teenage boy, and the town’s mood leans toward punishment and suspicion. In a desperate moment, Luke douses himself with gasoline and sets himself ablaze.
Cameron returns home to care for Seth as Ruth begins to break under the strain. While visiting Luke’s grave, Cameron meets Dolphin, and a complicated romance blooms, much to Seth’s horror. In a nearby barn, Seth and Kim uncover an ossified dead fetus, which Seth takes home, insisting it is Eben incarnate as a fallen angel. Cameron shows Seth a photograph of a baby dying from radiation poisoning, a discovery that both fascinates and unsettles him. The next day, Seth follows Cameron to Dolphin's house, where he witnesses Cameron emotionally confess to Dolphin his role in atomic bomb experiments. Dolphin and Cameron begin a clandestine affair; as Seth flees in terror, he witnesses the men in the Cadillac abducting Kim.
Cameron’s body begins to deteriorate from radiation sickness, which Seth attributes to Dolphin’s supposed vampirism. Kim’s body is recovered the next day, but authorities still suspect Luke’s death. As Dolphin and Cameron deepen their bond and contemplate running away together, Seth channels his anger toward Dolphin. That night, he consults with the fetus “angel Eben” on how to deal with her. The following day, when Dolphin is about to accept a ride from the Cadillac group, Seth does not warn her, and she is found dead soon after. Cameron breaks down in front of Seth, and the film ends with Seth running into an open field, shouting at the setting sun.
