I killed him...

Rachel Gordon wakes up from an induced coma following a car accident to the news that her much older husband, Anthony, has died at their home, and her immediate reaction is guilt.
The police also seem convinced she must have been involved. But Rachel can't actually remember any of the past month.
She has flashes of a man she met on her regular commute to work by tube. But there's no other evidence he exists. Did they have an affair? Could he have been responsible for Anthony's death? Or is Rachel guilty of something else altogether? And will she discover the truth in time to save herself?