While diving for an ancient Indian skull in a forested lake, Neve's oxygen tank ruptures and she quickly ascends from the watery depths. She returns to the shore and falls unconscious, stricken with decompression sickness. The next morning, Neve delivers the ancient skull to its proper place and travels to a clinic in Niagara Falls, where she begins to recover in a hyperbaric chamber.

Meanwhile, a young girl named Kiley experiences her first menstrual period while on a camping trip with her friend's family. Alienated and confused, Kiley wanders away from the campsite. Day turns to night and Kiley falls deeper into the woods, on a curious search for answers about her own body.

The two stories connect in the film's finale, a scene that unites Kiley's coming of age with Neve's search for the sacrificial relic. Niagara is a story of the feminine, a visual rendering of the symbiosis between bodily form and landscape by illuminating the liminal spaces between history, mortality, and sacrifice.