In 1952 the scientists at Porton Down were carrying out secret germ warfare trials off the coast of Lewis in the Outer Hebrides.
One of the germs they were testing was plague - the black death.
Experimental animals kept in cages on a pontoon were infected with a plague cloud released upwind of them.
One evening, as a trial was about to start, a trawler sailed unexpectedly into the danger zone.
Rather than stop the tests, the captain of the naval ship gave the order to continue, and the germs were released into the path of the trawler.
Jolyon Jenkins pieces together the events and the subsequent cover-up as the government tried to reconcile the conflicting demands of public health and official secrecy.
The programme use newly-declassified top secret documents, interviews with surviving witnesses, and the private diary of the chief scientist to shed light on a hitherto virtually unknown episode of the Cold War.