Leo attempts to help Mark Benson,whose wife Shona was imprisoned for killing their baby and begins to spot discrepancies in the expert witnesses at the trial. Petra Smith is abducted and imprisoned in a remote farm building at the same time as two women's corpses are discovered in a basement. The officer in charge, Chrissie Reid, once has an affair with Jack until he accused her of corruption in order to get an arrest. The two women have been injected with a toxic substance and Chrissie and Nikki visit the factory that makes it. The suspicious actions of scientist Lucas Ballinger cause them to follow him to the hide-out where a man called Hearns has imprisoned Petra and is about to inject her though Lucas is scared as he had not realised that anybody would die. Hearns kills him and sets fire to the building and Chrissie pursues him, leaving Nikki inside the burning building.

Nikki escapes the burning building but Petra dies and Leo's examination shows she was injected with anthrax. Jack identifies one of the basement victims,also injected,as being Lucas Ballinger's wife and Nikki surprises him at the scene of the murders though he flees. He goes after Hearns and later Chrissie finds Roper,who was the accomplice in taking the virus out of Ballinger's workplace, dead. The team must locate Ballinger and find out why he allowed his wife to be a victim as well as stopping him from committing a terrible revenge which could endanger thousands of lives. Leo meanwhile successfully gets Shona's conviction overturned.