Expo Lana Washington is giving a routine talk, having just returned from secondment, when an explosion rocks the heart of the city. But is she ready to return to active duty?

The police launch an investigation to find the bombers, whose skill and sophistication keeps them one step ahead. Under pressure to get results, Helen makes a breakthrough.

As the team mourns Thom's death, a new lead emerges - an informant, promising intelligence on the terror cell. Meanwhile a journalist is sent a suspicious package.