Three couples, friends since the university athletics team, travel to a house in the mountains for an unplugged weekend of drinking and bushwalking. No internet, no phones, no stress. On the first night, the topic of partner-swapping comes up. It's a joke - at first.
Not everyone is keen, but an agreement is made. The lights will be turned out. The three men will go into the three bedrooms. The three women will each pick a bedroom at random. It won't be awkward later, because no one will know for sure who they've slept with - though each guest is privately sure that they alone will be able to tell.
But when the lights come back on, someone is missing. A search of the house and the surrounding bushland turns up a body.
Five people remain. They have no way of contacting the police. Nor can they leave - the key to the car is missing. And the killer is just getting started.
Two weeks later, Senior Constable Kiara Lui drives to the house with her girlfriend, Elise. She's already interviewed the survivors. Now, she must work out which of them is lying, and why ...