Freshly installed in a country house with her husband and their 6-month-old son, Agathe discovers a child's bedroom that had previously been locked behind a partition. Untouched, nothing seems to have moved since the 90s. Even the old baby monitor abandoned in a drawer is still functional. Buoyed by her husband's enthusiasm, the young mother agrees to install their son there, at the price of hanging on to the device, listening for the slightest noise. But when strange emanations sow doubt and confusion in Agathe's mind, she will have to sort out her new environment, her latent fatigue and her maternal anxieties.
