Seventeen years after leaving the Lyell, Sam Ryan returns pleading for help. The shooting of the Health Secretary pulls the Lyell into a world of duplicity, intrigue and betrayal. Soon Nikki starts to suspect that Sam knows more than she's letting on. When the credibility of DNA - the bedrock of forensic science - is challenged, everything the Lyell holds dear comes under threat. Uncertain of who to trust, the team's investigation draws them into perilous confrontations as they fight to uncover the truth and protect everything they love.