The Time Lords' energy is being drained through a black hole, so all three Doctors are summoned to investigate. While the First Doctor advises them from the TARDIS scanner, the Second and Third Doctors travel through the black hole. They discover that Omega, a long-lost pioneer of Time Lord history, is imprisoned therein. Believing he has been abandoned, Omega -- now wielding supreme power over the anti-matter universe within the black hole -- is determined to destroy the Time Lords, and force the Doctors to remain within the black hole in his place.

For each of the first three seasons in which Jon Pertwee played the Doctor, Doctor Who had benefitted from strong “hooks” in each year's leadoff story: Pertwee's debut in Season Seven's Spearhead From Space, the introduction of the Master in Season Eight's Terror Of The Autons, and the return of the Daleks in Season Nine's Day Of The Daleks. Producer Barry Letts wanted to continue this tradition for the programme's tenth season, and felt that something to celebrate the show's anniversary was appropriate.

For quite some time, Letts had been encouraged -- both by fans and by others at the BBC -- to devise an adventure in which Pertwee's Third Doctor met his two predecessors, played by Patrick Troughton and William Hartnell. To date, Letts had dismissed this as being too fannish a notion, but in early 1972 he and script editor Terrance Dicks decided that a multi-Doctor story would be an excellent way in which to celebrate Doctor Who's decade on the air.

Dicks discussed the idea with Bob Baker and Dave Martin as the pair were completing work on The Mutants for Season Nine. On February 3rd, Baker and Martin submitted an idea called “Deathworld”. In it, the Time Lords are in conflict with a Federation of Evil led by a personification of Death. To avert all-out war, the Time Lords manage to convince the Federation to allow them to send the three Doctors into the Federation's Underworld domain. There, the Doctors will do battle against various realisations of Death -- including zombies, the Four Horsemen of the Apocalypse, the Hindu goddess Kali, and the cyclops Polyphemus from Greek mythology -- with the victor in the contests determining whether the Time Lords or the Federation of Evil will prevail.

Meanwhile, the Doctor Who production office contacted both Hartnell and Troughton to confirm their interest in returning to the series. Despite having been critical in the press of the programme's latter-day direction, Hartnell -- who had virtually retired from acting due to the debilitating arteriosclerosis which had been part of the reason he left Doctor Who in 1966 -- was quick to accept the offer. Troughton also agreed, but his schedule was so busy that he would not be available until November.

Consequently, it was decided that the anniversary story would be made second in the tenth recording block, making it the third Season Ten story to be taped after Carnival Of Monsters (completed as part of the ninth production block) and Frontier In Space. “Deathworld” therefore became Serial RRR. Pertwee was also approached to ensure he approved of the return of his predecessors, and gave his consent with the condition that his Doctor remain the adventure's focus.

Baker and Martin continued to work on their storyline during the spring of 1972. The main enemy became Ohm -- so named because “OHM” read upside-down is “WHO”, reflecting the writers' desire for him to be seen as the Doctor's opposite number. By mid-April, it became clear to the production team that Hartnell's health was too poor for him to take an active role in the entire serial. The First Doctor's involvement in the storyline would therefore have to be restricted to brief scenes which could be prefilmed. Dicks also asked Baker and Martin to include former companion Jamie McCrimmon (played by Frazer Hines) in the adventure, and suggested a romantic subplot involving Jamie and Jo Grant.

As the spring wore into summer, Letts and Dicks asked the Bristol Boys to make further changes to the storyline. Letts disliked the name Ohm (since the Doctor's real name was not “Who”) and so the central villain became Omega, named after the final letter of the Greek alphabet. Letts also felt that the “deathworld” setting was inappropriate for Doctor Who, believing that the limbo-like Underworld would be too incredible for the viewing audience. The setting was therefore changed to a planet on the other side of a black hole, and the monstrous Gell Guards (never named onscreen) were introduced. The scripts were finally commissioned under the title The Three Doctors on June 27th.

Baker and Martin rearranged Hartnell's role so that the First Doctor featured heavily only in episode four, when he joined his two successors for a final confrontation with Omega. After consulting with Hartnell's wife, Heather, however, Letts and Dicks realised that even this limited schedule would be too arduous for the actor, whose memory was becoming increasingly unreliable. Dicks therefore rewrote the scripts so that the First Doctor appeared only on the TARDIS scanner -- material which could be filmed in a single day. Hartnell and Troughton were both issued contracts on September 21st.

The story's title was changed to “The Black Hole” around the time that its director, Lennie Mayne, came on board. Mayne had helmed The Curse Of Peladon the year before. By early November, it was clear that Frazer Hines would be unable to take a major role in the story because of his obligations to the soap opera Emmerdale Farm. Sergeant Benton therefore took over Jamie's role -- without the romantic overtones -- though it was hoped for a time that Hines might still be able to tape a brief cameo appearance for the story's conclusion. When this proved impossible, it was suggested that Wendy Padbury might appear as Zoe Heriot. These plans were apparently scuttled by Pertwee, however, who was concerned that an abundance of old characters might detract from the show's current protagonists.

Production on “The Black Hole” began on November 6th, with one day at the Ealing Television Film Studios. This was spent on the fight between the Third Doctor and Omega's champion, as well as all of Hartnell's material. To compensate for the actor's failing memory, his lines were written on cue cards from which he could read as he was filmed. It transpired that this was Hartnell's final performance; he succumbed to his health problems on April 23rd, 1975.

Location work began on November 7th. Three sites in Rickmansworth, Hertfordshire were visited: the Summerfield Bungalow was Ollis' cottage, Springwell Reservoir served as the site where the weather balloon touched down, and Springwell Quarry posed as Omega's world. More filming at the quarry occurred on the 8th and 9th. November 10th saw cast and crew head to Halings House in Denham Green, Buckinghamshire, for scenes set outside UNIT headquarters. Two days of model filming then took place on the 14th and 15th at the BBC Television Centre Puppet Theatre.

Mayne originally intended to record one episode on each of his four studio days, but subsequently decided to modify this approach. For the first studio block, on Monday the 27th and Tuesday the 28th in BBC Television Centre Studio 1, Mayne taped all the scenes for the first two episodes (except the part two scenes inside Omega's stronghold) as well as the Time Lord sequences for the third and fourth installments. These days saw the debut of the new TARDIS set, designed by Roger Liminton. More faithful to Peter Brachacki's original design, it replaced the set Tim Gleeson had conceived for The Time Monster, which had warped in storage and had met with general disapproval. By now, the story's title had reverted to The Three Doctors.

The second studio block took place exactly two weeks later, on December 11th and 12th; this time the venue was TC6. Two and a half weeks later, the first episode of The Three Doctors led off Doctor Who's tenth season. The story was a milestone in more ways than one. With Doctor Who's ratings now stable after the ups and down of the late Sixties, Letts and Dicks had decided to use The Three Doctors as a vehicle for removing the Earthbound constraints which had been imposed on the programme for budgetary reasons at the end of the Troughton era. The Doctor was once again free to travel in all of time and space...

• 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 #260, 14th January 1998, “Archive: The Three Doctors” by Andrew Pixley, Panini UK Ltd.
• Doctor Who Magazine Special Edition #2, 5th September 2002, “Saturday Night's Alright For Fighting” by Andrew Pixley, Panini Publishing Ltd.