While lying in bed next to her husband one night, Claire Devon hears a low hum that he cannot. And, it seems, no one else can either. This innocuous noise begins causing Claire headaches, nosebleeds, insomnia, gradually upsetting the balance of her life, though no obvious source or medical cause can be found. When she discovers that a student of hers 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 neighbours who also perceive the sound. What starts as a neighbourhood self-help group gradually transforms into something far more extreme and with far-reaching, devastating consequences.