A threatening letter sent to the Prime Minister puts the British people in grave danger. The terrorist group John Amend-all are demanding that the government increases its aid to Africa, or they will release a deadly smallpox virus provoking a national epidemic.

The Smallpox Containment and Reaction Unit (SCRUB) operation is launched. Dr. Herry Smith is called away from his day job as Medical Director of Public Health to help. Teaming up with Detective Inspector Rebecca Hale, the pair combine their medical and investigative skills in a bid to crack down on the culprits.

But time quickly starts to run out, and before they know it hospitals start to fill with victims of the deadly virus.