Once you've heard them, you'll never forget them . . .

In the scorching summer of 1976 - the hottest since records began - 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 . . .