London. June 2019, early morning: on the foreshore of the River Thames a bag of bones is discovered. Human bones. DCI Samuel Owusu 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 are the seeds of a rare tree which lead DCI Owusu to a mansion in Chelsea where, nearly 30 years previously, three people lay dead in a kitchen while a baby waited upstairs...