The Massacre of St Bartholomew's Eve

From http://www.bbc.co.uk/doctorwho/classic/episodeguide:

The TARDIS materialises in Paris in the year 1572 and the Doctor decides to visit the famous apothecary Charles Preslin. Steven, meanwhile, is befriended by a group of Huguenots from the household of the Protestant Admiral de Coligny.

Having rescued a young serving girl, Anne Chaplet, from some pursuing guards, the Huguenots gain their first inkling of a plan by the Catholic Queen Mother, Catherine de Medici, to have all French Protestants massacred.

A further shock is in store for Steven, as it appears that hated Catholic dignitary, the Abbot of Amboise, is actually the Doctor in disguise. Held responsible for the failure of a plot to assassinate de Coligny, the Abbot is executed by the Catholic authorities and his body left lying in the gutter.

To Steven's relief, it transpires that the Abbot was not the Doctor after all, but merely his physical double. The two time travellers meet up again at Preslin's shop, where Steven has gone in search of the TARDIS key, and regain the safety of the ship just as the massacre begins.

The TARDIS then lands on Wimbledon Common in 1966 and the Doctor and Steven gain a new companion, Dorothea 'Dodo' Chaplet.

14 out of 78 (17.9%) raters say this story requires a previous story.