The murder of a promising boxer draws Nikki and Jack into the world of underground boxing. Meanwhile, Jack's brother Ryan is released from prison and comes to live with Jack and Conor, leaving Jack struggling to maintain peace with his brother and keeping the secret that he might be Cara's father. When Jack and Nikki's investigation hits something of a brick wall, they are introduced to forensic ecologist Simone. Her unique skills and lively personality prove to be exactly what the Lyell team need.

Nikki's students make an alarming discovery while dissecting a body donated for their training. The case that follows brings Nikki close to student Ollie, stirring dangerous feelings. Meanwhile, Jack and Simone investigate the care home where the victim died, but find themselves fighting to keep the residents alive when a diverted river floods the area.