Nikki and Jack strive to track down a sniper at large in London and stem a public crisis.

Nikki and Jack investigate links between the victims to close the net on the killer.

Nikki and Jack investigate a man killed by a train on the London underground.

Nikki helps high-flying DI Luke Nelson unearth painful childhood memories.

A suspected paedophile is found murdered in a children's playground.

Nikki, Jack and Thomas focus in on Lizzie's movements on the night of Barge's murder.

The nanny of a Russian oligarch is killed in a botched assassination attempt.

The Lyell team are under pressure to catch the killer of an admired Essex police officer.

When the body of local criminal Jason Simons is found, the course of the case is changed.