The TARDIS lands in Paris on 19th August 1572 and the Doctor, driven by scientific curiosity, leaves Steven in order to meet and exchange views with the apothecary Charles Preslin. Before he disappears, he warns Steven to stay out of 'mischief, religion and politics'.
Ignoring the Doctor's warning to stay out of trouble, Steven finds himself caught up with a group of Huguenots. The Protestant minority of France is being threatened by the Catholic hierarchy and danger stalks the streets of Paris.
As Steven tries to find his way back to the TARDIS, he is horrified to find that the greatest persecutor of the Huguenots is actually none other than the Doctor himself.