34-year-old psychotherapist Siri Bergman is terrified of the dark.
Living alone in a picturesque, but isolated area east of Stockholm, she has tried hard to convince herself that she has moved on since her husband, Stefan, died in a diving accident several years ago. But when she goes to bed, Siri leaves all the lights on unable to shake the feeling that someone is watching her at night.
So when one night she wakes up to find that the house is pitch black, and the torch she keeps by her bed for back-up is not where she thought she'd left it, it seems that Siri's worst fears have been realized.
And when the lifeless body of Sara Matteus, one of her patients, is found floating in the water near Siri's house, events quickly spiral. It is clear that Siri is in great danger, and she is thrown headlong into the centre of a murder investigation which will put each of her closest friends under the spotlight and force her to relive her troubled past.