Tracy, head of Wiltshire Council Public Health, must handle the fallout after two people fall unconscious in a city center due to an unknown illness. She discovers that it's actually a deadly poison that's been spread all over Salisbury.

The city falls further into lockdown as public panic increases and decontamination efforts expand. Tracy continues her desperate search for the point of origin of the poison, furthering the strain on her deteriorating relationship with her family.

The decontamination efforts finally show results and eventually life in Salisbury returns to normal. However, a mistake with good intentions creates a resurgence of the poison, causing Tracy's life to spiral just as she was beginning to fix it.

As the city struggles to cope with the renewed panic, Tracy learns that the contamination radius might be even bigger than last time. The second poisoning outbreak is much worse than the first, the random discovery means the poison could be anywhere.