As the basement women case highlights two cooperating killers, Nikki, Jack and DI Reed realise one victim was killed by a controlled sedative, stolen from a high-tech vaccine research lab. And suspicion falls on Lucas Ballinger, a scientist who works there. Meanwhile, Leo draws a blank with his theory that Shona's baby died of natural causes, but when he discovers Mark's violent past, Leo fears Shona may have taken the rap for her partner's child abuse.
Jack makes a chilling forensic breakthrough which exposes a dangerous plot to breach national security, leaving the Lyell Centre paralysed by a government-imposed lockdown. With Nikki, Jack and DI Reed locked in a race against time to find Ballinger and prevent mass casualties, Leo has one last shot at proving that Shona's baby died from natural causes in a desperate bid to reunite her and Mark with their other son before he is adopted.