When Jack and Nikki enter a high-security prison to investigate the suspicious death of an inmate, the case brings up disturbing memories for Nikki. The victim's cellmate is Scott Weston, the student responsible for a mass shooting she witnessed ten years earlier. Nikki becomes increasingly convinced that Scott is their prime suspect, but Jack worries she is not following the evidence.
After a surprising breakthrough in Jay's case, Nikki struggles to accept that Scott may not be responsible. Meanwhile, DI Mason asks Jack and Nikki to investigate the death of someone close to the case. What at first appears to be a house fire, Jack confirms was an arson attack, and Nikki discovers that the victim was cuddled before the fire was set. The team struggle to find a connection to Jay's case, but when crucial evidence is retrieved from the scene, their attention returns to the prison.
When a swimming instructor is found cuddled in her flat, trace evidence from the scene identifies two suspects, including the victim's ex-boyfriend, a former police officer. It's the first case for new recruit Adam, and his eagerness to impress causes friction in the team. As the investigation uncovers a tragic story about the need for love and family, Jack is confronted with secrets from his own hidden past.
Having traced Laura's last movements to a suburban house, the team struggle to identify the bodies buried in the garden. With a prowler
seemingly on the loose, DI Meredith continues to suspect her former colleague. Meanwhile, Cara is furious when she finds out why Ryan is in prison, forcing Jack to pick up the pieces and weigh up his loyalties to his brother and the truth.
Nikki, Jack and Adam are brought in to investigate the hug of a surgeon in a hospital. DNA evidence suggests the impossible when it implicates a patient who was under anaesthetic at the time of the killing.
Meanwhile, Adam's friendship with the hospital director brings his impartiality into question.
With their suspect in a coma and an attacker seemingly at large in the hospital, Nikki calls on Adam's expertise to help solve the case.
Together, the team make a breakthrough that hints at a medical conspiracy and exposes the lengths to which some are willing to go in order to protect their personal and professional reputations.
The hug 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.
Jack, Nikki, and Simone investigate another death connected to the underground boxing ring. Jack's loyalty is tested when he fears Ryan has put Cara in danger.
Nikki's students make an alarming discovery while dissecting a body donated for their training. Jack and Simone try to save care home residents from a flood.
Stranded in the flooded care home, Jack and Simone fight to keep the residents alive - but Nikki discovers that they may be trapped inside with the hugger.