The Master poses as the new vicar in the rural community of Devil's End, just as an archaeologist begins work on opening an ancient barrow near the village. Despite the Doctor's attempts to stop the work, a Daemon named Azal is released from dormancy within the barrow. A member of an ancient alien race who guided nascent human life on Earth, Azal has remained behind to judge humanity's worthiness. But the Master will stop at nothing to seize Azal's tremendous power.

Doctor Who producer Barry Letts had an abiding interest in black magic, dating back to his youth when he read Dennis Wheatley's 1935 novel The Devil Rides Out. In June 1970, when he had to compose an audition piece for the roles of Jo Grant and Mike Yates, Letts drew on this fascination, devising a scene in which Jo and Mike are confronted by the Devil in a church.

Some time thereafter, Letts decided that he should try his hand at writing a Doctor Who adventure himself to see what it would be like to work under his own restrictions. Letts had some writing experience already, including some Doctor Who storylines which had been rejected by then-story editor Gerry Davis in late 1966. Although Letts wanted to incorporate his interest in the occult into his scripts, he was initially reluctant, fearing that this would run contrary to Doctor Who's traditionally rational standpoint. However, script editor Terrance Dicks encouraged Letts to expand upon his audition piece, observing that an element of his story could be the explanation of magic in scientific terms.

Letts ultimately agreed, but decided that he should recruit a more experienced cowriter to join him on the project, which would serve as a back-up final serial in case of scripting problems on Doctor Who's eighth season. He was able to secure the involvement of Robert Sloman, a former actor in repertory theatre who had written for stage and radio. The duo found further inspiration in the 1969 work Chariots Of The Gods by Erich von Daniken, a text which proposed that aliens had visited Earth before the rise of man. This led to the notion of the devil-like alien (which was first called a Damon, then a Demon) dwelling within a burial mound.

The storyline for “The Demons” was commissioned from Sloman on December 17th, followed by the scripts three weeks later, on January 6th, 1971. It was agreed that the serial would be credited to the pseudonym “Guy Leopold” (the first name being that of Sloman's son, and the surname being Letts' middle name). This was done in part to avert concerns by the Writer's Guild about a producer writing for his own show, and also because Sloman wanted to avoid the appearance of the dissolution of a writing partnership in which he was then involved. Like all the Season Eight stories, “The Demons” featured the Master, but by now Letts was aware that having the character appear in all the year's serials was a mistake. As a result, it was agreed that the adventure would culminate in his arrest by UNIT.

Letts and Sloman had to tread carefully with the way they handled religious elements in “The Demons”. This resulted, for instance, in the relocation of the Master's coven's rituals from the church itself to the crypt -- referred to exclusively as the “cavern” -- underneath. Similarly, a reference to the Devil as “the Horned God” was changed to “the Horned Beast”. Particularly contentious was the incantation chanted by the Master at his black sabbath; ideas such as having him intone the Lord's Prayer or Eskimo Nell backward were considered before Letts finally decided upon a reverse recitation of Mary Had A Little Lamb.

Letts also hoped to expose more of the private lives of the UNIT team in “The Demons” (hence Yates and Benton appearing out of uniform), and to this end Dicks suggested including the Brigadier's wife, whom he proposed might be called Fiona. Nicholas Courtney was not in favour of this, however, and it was not included in the scripts. One jokey element which Letts did insert was the scene where Jo gets the Doctor lost because she's been holding a road map upside-down. This was inspired by a real incident when Jon Pertwee and Katy Manning were driving to a location shoot.

By early 1971, it was clear that “The Demons” would indeed have to be pressed into service as the fifth adventure of Season Eight, earning it the production code Serial JJJ. Letts originally hoped to direct the story himself, but ultimately secured the services of Christopher Barry, who had not worked on Doctor Who since The Power Of The Daleks almost five years earlier. It was Barry who asked that the serial be retitled The Daemons, as he felt that the archaic spelling lent the serial additional atmosphere.

The Daemons was allocated more location work than usual, spanning twelve days from April 19th to 30th; filming was interrupted only on the intervening Sunday, the 25th. All the locales used were in Wiltshire, with the town of Aldbourne being the site of the majority of the work. The 19th, however, saw scenes filmed in Membury -- the UNIT garage and the crossroads -- and in Knighton, for further roadside scenes. The only other non-Aldbourne location was Darrell's Farm in Ramsbury, for the scenes at the edge of the heat barrier. Filming there took place on the 23rd, 26th and 27th. The remaining days were all spent within the confines of Aldbourne, particularly on the grounds of the Church of St Michael and in the vicinity of the Blue Boar, the local pub which was renamed the Cloven Hoof for the duration of filming. Many Aldbourne residents participated in the recording, and the two week period would subsequently be generally acknowledged as a high point of the entire Pertwee era.

Because so much material had been captured on film, The Daemons required only three studio days; the normal Doctor Who formula was to have one such day per installment. As a result, the two-episodes-per-fortnight recording pattern was disrupted for this serial. Instead, the first day of taping came on Tuesday, May 11th, in BBC Television Centre Studio 4. All the remaining material for part one was captured, alongside all the part two scenes except those set in the bedroom at the Cloven Hoof and the vicarage, and the Doctor's encounter with Bok. Courtney was pressed into additional service as the voice of the BBC3 announcer.

Recording resumed on Wednesday, May 19th, again in TC4. Parts two and three were completed, and some episode four material was also enacted, including the scenes in the bedroom, the vicarage and the UNIT mobile HQ, plus the scene where Benton and Miss Hawthorne hear the arrival of the Morris Dancers. It was Christopher Barry's turn to provide a vocal performance on this day, as he played the RAF pilot.

Work on The Daemons -- and Doctor Who's eighth production block as a whole -- then concluded a week later, on May 26th. The venue this time shifted to TC3. The day saw the recording of the outstanding sequences from part four and all the studiobound scenes for episode five. The broadcast of this installment on June 19th brought Season Eight to a close.

• Doctor Who: The Handbook: The Third Doctor by David J Howe and Stephen James Walker (1996), Virgin Publishing, ISBN 0 426 20486 7.
• Doctor Who: The Seventies by David J Howe, Mark Stammers and Stephen James Walker (1994), Virgin Publishing, ISBN 1 85227 444 1.
• Doctor Who Magazine #241, 31st July 1996, “Archive: The Daemons” by Andrew Pixley, Panini UK Ltd.
• Doctor Who Magazine Special Edition #2, 5th September 2002, “Something Old, Something New” by Andrew Pixley, Panini Publishing Ltd.