Early morning, June 2019: on the foreshore of the river Thames, a bag of bones is discovered. Human bones.

DCI Samuel Owusu is called to the scene and quickly sends the bag for forensic examination. The bones are those of a young woman, killed by a blow to the head many years ago. Also inside the bag is a trail of clues, in particular the seeds of a rare tree which lead DCI Owusu back to a mansion in Chelsea where, nearly thirty years previously, three people lay dead in a kitchen, and a baby waited upstairs for someone to pick her up.

The clues point forward too to a brother and sister in Chicago searching for the only person who can make sense of their pasts. Four deaths. An unsolved mystery.