One evening, a wife asks her husband a question: who else would you go for, if you could? It is a simple question - a little game - that will destroy her life. Carly and Rob are a perfect couple. They share happy lives with their children and their close friends Craig and Jenny.

They're lucky. But beneath the surface, no relationship is simple: can another woman's husband and another man's wife ever just be good friends? Little by little, Carly's question sends her life spiralling out of control, as she begins to doubt everything she thought was true. Who can she trust? The man she has promised to stick by forever, or the best friend she has known for years? And is Carly being entirely honest with either of them? Obsession is a dark, twisting thriller about how quickly our lives can fall apart when we act on our desires.