The story begins in 1943, in a wintry Nissan hut laboratory somewhere in the English countryside. A scientist anxiously awaits the decision of the War Cabinet on the use of a deadly virus - any one of the Four Horsemen as they've nicknamed the toxins could defeat the Nazis. Word arrives that Churchill has forbidden the use of any biological weapon unless the Allies are in certain defeat...
Forty years later Arthur and Tony are at work deep in the sewers beneath London searching for a blockage somewhere beneath the city. They find instead an old bunker and an unexploded bomb from WWII. A collapse in an old sewer wall has set the bomb off ticking. And Tony has found a small glass phial in a desk drawer in the bunker, then he drops it. Then the bomb goes off, causing the tunnels of the nearby Underground to cave in just as the train approaches Moorgate station. On the train is Detective Sergeant Andy Wise who leads the survivors in his carriage to safety.
Over the coming days Londoners begin to commit irrational acts of violence and then have no memory of doing so. Others break out in skin rashes and seeping sores. Andy Wise starts to piece together a Government cover up. He traces the survivors of the Tube cave in but finds them missing and their houses inexplicably burned down. He learns that others are being taken away in secret to an army base in the country... It turns out the scientist during the War had developed a strain of the anthrax virus. Of the four cultures created, Diabolus was the most virulent and dangerous, which was why it was never used. When Churchill vetoed it's use, the scientist absconded with one of the phials, went to the bunker in London just as a German bomb dropped on it, where it lay until the sewer workers found it.