Claire is a popular English teacher who starts hearing a sound that neither her husband nor her daughter can hear. This seemingly innocuous hum triggers migraines, nosebleeds and insomnia, with no obvious source or medical explanation. After weeks of suffering, the strain starts to fracture her relationships with family, friends and colleagues.

When she discovers that a student of hers can also hear the hum, the two strike up an unlikely and intimate friendship that gradually transforms into something more extreme, with devastating consequences.