A young couple wakes disorientated, trapped and without food or water. There's no escape. Instead there's a gun loaded with a single bullet and a mobile phone with charge enough only to deliver a short message: when one of you kills the other, the survivor will walk free

Other pairs are soon taken. The victims appear to be chosen at random, but the planning is meticulous. And until DI Helen Grace can establish a connection, a twisted killer will remain out of reach. A breakthrough is elusive, but then the investigation begins to turn full circle . . .