When John Briggs, a wealthy business owner, is found dead, Nikki works with forensic scientist Jack Hodgson to convince DI Gold it was murder. Leo, warned by the Home Office to rethink the Lyell Centre's operations, helps Nikki prove Briggs died from a mysterious allergic reaction. However, as suited sharks circle to buy Briggs' ailing company, Jack believes someone exploited this allergy to kill.

Briggs' estranged daughter Geraldine is left the company in his will, yet she is shocked that he had tracked down her long lost daughter Deanna, and the pair make up for lost time. But when Deanna's body is discovered in the Thames, Nikki, Jack and DI Gold are sure the murders are linked, as Leo decides how to safeguard the Lyell Centre's future - employing Jack to launch a forensics department. As the new team are cemented, evidence from Deanna's body provides a vital lead on the killer's motive.

The case of two young women shot dead in a house basement forces Jack to lock horns with an old flame, the ambitious DI Chrissy Reed. As the pair's animosity rapidly escalates due to their conflicting motive theories and failure to ID the victims, Nikki's probing unearths the potentially career-damaging truth behind it.

Meanwhile, Leo wrestles with providing a second post-mortem on a baby suspected of being abused and smothered to death by his mother, Shona. The father, Mark, is adamant she is innocent. He pleads with Leo to find an alternative cause for their son's fractures and death to free his wife before their other son, Darius, is adopted. However, Leo soon realises that to achieve this he must re-examine the work of his pathologist friend, Richard Fell, and prove he made a mistake in his evidence against Shona.