GP Mona Harcourt hears that her brother, Kareem, has been kidnapped on his way home to the UK from his charity clinic in Lahore. It all unravels on the same day that a terrorist attack brings London to a standstill.

After contacting Danny in Pakistan, Mona heads to Lahore to identify Kareem's body. The trip provides her a chance to make contact with Danny but also leads her into danger.

Despite her injuries, Mona heads back to the UK intending to investigate Danny's potential links to radical Islam. Her attempts to get Danny to safety on the day of Kareem's funeral results in unexpected consequences for the family.

After Mona is released from custody, she agrees to be a tasked witness to help police bring in Danny as an informant. The outcome of her actions at home is devastating, with her marriage almost at tipping point.

In an attempt to save Danny, Omar puts the entire counter-terrorism operation in danger. With the terror cell compromised and off-grid, the police's trust in Danny begins to falter.

Barnes and the counter terrorism team race against the clock to prevent imminent attacks, while Mona and Danny remain trapped in the terror cell with their lives on the line.