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.

When a swimming instructor is found murdered in her flat, trace evidence from the scene identifies two suspects, including the victim's ex-boyfriend, a former police officer. 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 murder 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 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.

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. 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.

Stranded in the flooded care home, Jack and Simone fight to keep the residents alive. Meanwhile, Nikki and detective Galloway join the emergency relief effort and discover that Jack may be trapped with the killer. Shaken by events, and by Nikki's relationship with her student, Jack takes matters into his own hands.