IN THEIR SLEEP follows a mourning mother after the brutal death of her 18 year-old son. Sarah's life is in pieces. When late one night, her car accidentally hits Arthur, a young man the same age as her boy, who suddenly emerges out of the forest and onto the road, Sarah sympathizes with him, taking him in, only to be tracked down by the burglar whose murderous rage towards Arthur forces her to take action. Unbeknownst to Sarah, things aren't quite the way they seem... but by the time she finally realizes, it is too late to turn back.