One night, while lying in bed next to her husband, Claire Devon suddenly hears a low hum. This innocuous sound, which no one else in the house can hear, has no obvious source or medical cause, but it begins to upset the balance of Claire's life. When she discovers that one of her students can also hear the hum, the two strike up an unlikely and intimate friendship. Finding themselves increasingly isolated from their families and colleagues, they fall in with a disparate group of people who also perceive the sound. What starts out as a kind of neighbourhood self-help group gradually transforms into something much more extreme, with far-reaching, devastating consequences.