Deep foundations are being dug on a City building site when some old brickwork is revealed. Part of it collapses, releasing a gust of air so foul that the men nearby begin to choke and retch.
Finally the nauseous stench disperses, and they discover a burial crypt dating back to the Great Plague of London in 1665. But these pits should never be disturbed.
When the intruders begin to show horrifying symptoms - fever, nausea, revolting swellings - all will die hideously within five days. The frightened authorities struggle to stop a national crisis. So the victims are isolated, and a cover-up is launched.
But one of the men who entered the pit cannot be traced.