The TARDIS lands in 1572 Paris. While the Doctor visits an apothecary named Charles Preslin, Steven befriends two Huguenots in the service of Admiral de Coligny, who is loyal to France's Protestant prince, Henry of Navarre. They meet Anne Chaplet, a servant employed by the Abbot of Amboise. Anne reveals that the Abbot is helping plan a slaughter of Huguenots: a massacre sanctioned by Catherine de Medici, the King's mother. But when Steven discovers that the Abbot looks just like the Doctor, he must determine whether this is really his friend in disguise -- and, if so, the role he must play in the terrible events.

On February 24th, 1965, almost a year to the day since he was commissioned to write The Aztecs, John Lucarotti received an invitation from story editor Dennis Spooner to submit new ideas for Doctor Who. The two men met on the 26th, and agreed that India would be an appropriate setting for an historical adventure. No Doctor Who serial had yet been situated on the subcontinent although, in late 1963, Terry Nation had briefly developed a storyline called “The Red Fort”, which would have taken place during the Indian Mutiny of 1857. On March 22nd, Lucarotti discussed his new project with Waris Hussein, who had directed the writer's first Doctor Who serial, Marco Polo. Hussein also recommended the Mutiny as an event in Indian history which could form a suitable basis for an adventure. Unfortunately, Lucarotti then learned that historical serials in Doctor Who were restricted to settings prior to the sixteenth century. Spooner advocated for an exception to be made, but without success.

Spooner instead gave Lucarotti free reign to select an alternative historical period. The writer began composing a storyline about the Vikings' first successful crossing of the Atlantic Ocean to the island of Newfoundland, which occurred around the year AD 1000. As Lucarotti was developing his ideas during April and May, Donald Tosh replaced Spooner as story editor. Tosh contacted Lucarotti to discuss ideas for Doctor Who, unaware that he already had a work in progress. Following a meeting in late May, Tosh indicated that Lucarotti should proceed with the Viking storyline. On June 8th, Lucarotti met with both Tosh and incoming Doctor Who producer John Wiles. The writer agreed to make some changes to reflect the more mysterious atmosphere the new production team envisaged for the series, but was otherwise confident that a commission would follow. As such, on June 24th, Lucarotti was astonished to learn that his storyline had been deemed unsuitable.

John Wiles wanted Doctor Who to explore religious conflict

Frustrated that two proposals had now been rejected after what he had considered to be Spooner's guarantee of a paid serial, Lucarotti involved his agents. To resolve the matter, Tosh agreed to work with Lucarotti to develop a new storyline that would meet the production team's needs. Wiles wanted Doctor Who to explore religious conflict, which he felt would draw a dramatic contrast to the more whimsical storylines explored during the programme's first two seasons. It was apparently Tosh who specifically suggested the 1572 Huguenot massacre as a potential setting.

In the second half of the sixteenth century, France was roiled with religious turmoil, particularly between the Catholic majority and the Huguenots, who followed Calvinist Protestantism. Tensions were exacerbated by the perceived influence that the Huguenots had on the French King, Charles IX; a prime example was his close friendship with Admiral Gaspard de Coligny, the Protestants' political leader. On August 18th, 1572, the queen mother, Catherine de Medici, tried to broker peace by marrying her Catholic daughter, Margaret of Valois, to King Henry of Navarre, a prominent Huguenot. However, the manoeuvre was widely opposed by the Catholic community. Following a failed attempt to assassinate de Coligny on August 22nd -- which may or may not have been arranged by Catherine -- fears of Huguenot retaliation grew. On St Bartholomew's Eve (August 23rd), Catherine convinced Charles to act swiftly to assassinate the Huguenot leadership, including de Coligny. A wave of popular anti-Protestant violence was incited, which soon spread from Paris to the rest of France. It's likely that five to ten thousand Huguenots were killed in the weeks that followed. The broader religious conflict would persist until 1598, when Henry of Navarre ascended to the French throne as Henry IV. This was facilitated by his conversion to Catholicism, but he worked quickly to safeguard the Huguenots by issuing the Edict of Nantes, which guaranteed their rights and freedoms.

Lucarotti was commissioned for “The War Of God” on July 9th. He wrote efficiently, delivering his scripts in less than two weeks. Lucarotti drew on the expressed interest of both Wiles and William Hartnell to have the star portray a role other than the Doctor. Indeed, at one point, Hartnell had suggested that he could play the Doctor's evil time-travelling progeny in a story called “The Son Of Doctor Who”! This led to the invention of the Abbot of Amboise, who was named for a town in central France which was the site of a failed 1560 Huguenot conspiracy against the powerful Catholic House of Guise.

As the summer progressed, Wiles and Tosh realised they had a problem on their hands regarding the Doctor's female companion. Having decided to dispense with Vicki and replace her with the Trojan handmaiden Katarina in The Myth Makers, they now conceded that the new character's antique origins would pose difficulties for writers. It was therefore decided that Katarina should be killed off in the serial which would precede “The War Of God”, The Daleks' Master Plan. As a result, it appears that Lucarotti was asked to revise his scripts to incorporate a new companion: a young Huguenot maid called Anne Chaplet. Anne would escape the massacre and join the Doctor and Steven in the TARDIS.

Upon learning of the extent of Donald Tosh's changes, John Lucarotti asked that his name be removed from the credits

By the autumn, Wiles and Tosh were still dissatisfied with “The War Of God”. Keen to avoid upsetting Lucarotti any further, they decided to pay him for his efforts, with the intention that Tosh would perform all additional rewrites. Upon learning of the extent of Tosh's changes, Lucarotti asked that his name be removed from the credits, although this request was eventually rescinded. Tosh took the opportunity to address what he felt to be unacceptable deviations from the historical record. He also rearranged the pace of events so that Hartnell would need to appear as both the Doctor and the Abbot in the first episode only, thereby simplifying the logistics of the production. Hartnell's lone scene for episode two -- which, like the third installment, saw him exclusively played the Abbot -- would be pre-filmed, enabling him to take a week's holiday.

Most significantly, Wiles and Tosh realised that adopting Anne as a companion would present many of the same difficulties they had encountered with Katarina. Tosh therefore rewrote the serial's ending to have Anne remain in sixteenth-century France, with the implication that she may have survived the massacre by acting on the Doctor's advice. A coda was added, set in modern-day England, introducing a new character devised by Wiles and Tosh. This was Dorothea “Dodo” Chaplet, by inference a descendant of Anne's, who would become the Doctor's companion.

By late 1965, the story had acquired a new title, The Massacre Of St Bartholomew's Eve, after the shorter “The Massacre Of St Bartholomew” had been inconsistently applied from around August. For the first time, a Doctor Who serial would be directed by a woman: Paddy Russell. This had nearly happened much earlier; Russell had been assigned to Inside The Spaceship in early 1964, but proved unavailable for the required dates.

In early December, Jackie Lane was cast as Dodo. In the summer of 1963, Lane had been under consideration to play the Doctor's grandchild, Susan, but she was uninterested in committing to a long-term assignment at the time. Wiles was familiar with her work on his play Never Had It So Good, which had been staged at the Library Theatre in her native Manchester in 1961. On December 29th, Lane was contracted to play Dodo for thirteen episodes, spanning the final part of The Massacre Of St Bartholomew's Eve through The Gunfighters, with an option for twelve more. Wiles hoped that Dodo could be made distinctive by having Lane's long hair styled differently in every story. However, this plan was scuppered when Lane opted for a very short haircut soon after passing her audition.

As 1966 dawned, production on The Massacre Of St Bartholomew's Eve got under way with filming at the BBC Television Film Studios in Ealing, London. This took place from January 3rd to 6th, concentrating on material set on the streets of Paris. On the last day, Hartnell (as the Abbot of Amboise) performed the only scene required of him for The Sea Beggar. On January 7th, a small amount of location filming for Dodo's introductory scene was conducted on Windmill Road, near Wimbledon Common in Wimbledon, London. It had been hoped that William Russell and Jacqueline Hill might make cameo appearances as former companions Ian and Barbara on this day; hearing the TARDIS dematerialising, they would have just missed seeing it as it disappeared. In the event, the pair were replaced with a woman (played by Marguerite Young) walking her dog. The location filming marked Jackie Lane's first work on Doctor Who, and a photocall was held to introduce her on the same day.

Meanwhile, both Wiles and Tosh had resigned from Doctor Who after encountering numerous difficulties during the making of The Daleks' Master Plan. Tosh was the first to leave the programme, handing over to Gerry Davis in mid-January 1966. Davis' first credit would come on the fourth episode of The Massacre Of St Bartholomew's Eve; as a result, Tosh could receive a co-writing credit on this installment, something typically forbidden for a BBC story editor.

Studio recording for The Massacre Of St Bartholomew's Eve began on January 21st, and continued on consecutive Fridays; Doctor Who returned to its usual production home of Riverside Studio 1 in Hammersmith, London after having relocated to BBC Television Centre for all twelve episodes of The Daleks' Master Plan. Hartnell was absent from the recording of The Sea Beggar on January 28th while he enjoyed his week's vacation. Jackie Lane made her first appearance in the Doctor Who studio during the closing moments of Bell Of Doom on February 11th. The broadcast of The Massacre Of St Bartholomew's Eve saw Doctor Who return to its original 5.15pm timeslot, swapping places with Juke Box Jury. This meant that it was now once again preceded by Grandstand, and followed by a news update and the music showcase.

• Doctor Who Magazine #233, 20th December 1995, “Archive: The Massacre Of St Bartholomew's Eve” by Andrew Pixley, Marvel Comics UK Ltd.
• Doctor Who Magazine Special Edition #7, 12th May 2004, “I'm Into Something Good” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #7, 2018, “Story 22: The Massacre Of St Bartholomew's Eve”, edited by Mark Wright, Hachette Partworks Ltd.
• Doctor Who: The Handbook: The First Doctor by David J Howe, Mark Stammers and Stephen James Walker (1994), Virgin Publishing.
• Doctor Who: The Sixties by David J Howe, Mark Stammers and Stephen James Walker (1992), Virgin Publishing.