A car bomb explodes in Liverpool and Zoe is sent undercover to investigate. Tom's life gets complicated when he falls in love with a woman he met while working undercover.

The team uncovers connections between racist ring-leaders, politicians and mass killings of immigrants. The gang has sophisticated equipment to protect itself from electronic infiltration. Tom is forced undercover, and recent recruit Helen pretends to be his wife.

Working undercover, Zoe attends a function at the State Consulate of Turkey. Whilst still inside, the building is dramatically stormed by heavily armed Kurdish freedom fighters. Led by the beautiful Leyla Ahmed (Katie Jones), the terrorists demand the release of fellow paramilitaries back home.The Kurds use the threat of violence to impose a siege and, for now, they’re in control.

Ellie starts to doubt whether she can cope with dating a spy. Tom's mentor is working undercover, but his methods start to appear dangerously unconventional. Meanwhile, Zoe discovers Tessa is up to something.

It’s a media scramble when ex-MP Hampton Wilder (Tim Pigott-Smith) is released from prison, having served a sentence for embezzlement. He is swiftly driven away in a car, accompanied by Danny: Wilder has requested an urgent meeting with MI5.

Tom is confronted by an Irish terrorist who seems to be trying to help MI5, but nothing is as simple as it seems. Ellie and Maisie move into Tom's house, just as he's tricked into bringing a bomb home in a laptop.