In the scorching summer of 1976—the hottest on record—Christopher Norton, his wife Laura and their young daughter Faye settle into their new home in north London.

The faded glory of the Victorian house is the perfect place for Norton, a composer of film soundtracks, to build a recording studio of his own. But soon in the long, oppressively hot nights, Laura begins to hear something through the crackle of the baby monitor. First, a knocking sound. Then come the voices.

For Norton, the voices mark an exciting opportunity. Putting his work aside, he begins the project of a lifetime—a grand symphony incorporating the voices—and becomes increasingly obsessed with one voice in particular. Someone who is determined to make themselves heard . . .