Mike Yates summons Sarah to a meditation centre, where he believes a man called Lupton is up to no good. In fact, Lupton has gained fantastic powers after bonding with one of the Spiders who rule the planet Metebelis Three. Their mission is to find the blue crystal which the Doctor stole from Metebelis Three, and recover it on behalf of the mammoth Great One. Sarah is teleported to the Spiders' planet, where she finds a regressive human colony living in fear. The Doctor has no choice but to follow her -- setting the stage for an encounter with the Great One that will change his life forever.

Roger Delgado had played the Master on Doctor Who since 1971 but, after appearing in all five of that year's serials, he had made only sporadic appearances during 1972 and 1973 -- a decision implemented by the production team to avoid overexposing the character. Despite this, Delgado found that many casting directors were operating under the assumption that he was still fully committed to Doctor Who, and were passing him over for work. As such, during the making of Frontier In Space, he and producer Barry Letts agreed that the Master would be written out of Doctor Who in the final serial of Doctor Who's eleventh season, in 1974.

To compose the Master's swansong, Letts turned to Robert Sloman, with whom he had collaborated on the last three season finales, most recently The Green Death. Although he was beginning to tire of Doctor Who, Sloman accepted a commission for a storyline entitled “The Final Game” on February 15th, 1973. This adventure was to culminate in an ambiguously self-sacrificial death for the Master, which resulted in the Doctor's life being saved. Unfortunately, “The Final Game” had to be abandoned following Delgado's June 18th death in a car accident.

The loss of Roger Delgado became a catalyst for the break-up of the Doctor Who team

The loss of the beloved actor became a catalyst for the break-up of the team which had charted Doctor Who's course for almost half a decade. By the time 1973 entered its final weeks, it was known that Letts, script editor Terrance Dicks, and star Jon Pertwee would all be leaving Doctor Who following Season Eleven. Sloman was therefore asked to deliver a new storyline under his original commission, which would bring the Third Doctor's era to a close. As before, Sloman worked closely with Letts and, as with “The Final Game”, they decided to draw upon Eastern philosophies with a narrative in which the Doctor confronted his destructive and rapacious thirst for knowledge. Originally called “The Planet Of The Spiders”, the title was shortened to simply Planet Of The Spiders by the time Sloman's scripts were commissioned on December 5th.

In developing the adventure, Letts was keen to reward Pertwee -- who loved gadgets and vehicles of all kinds -- with an extended chase scene; this would form the bulk of Episode Two. Letts also saw Planet Of The Spiders as an opportunity to revisit Mike Yates, formerly of the United Nations Intelligence Taskforce (UNIT). The character had betrayed his friends in Invasion Of The Dinosaurs, earlier in Season Eleven, but Letts had decided against killing Mike in order to allow for a redemptive arc. It was also decided that a subplot concerning the planet Metebelis Three -- which had lightly run through Season Ten -- should be wrapped up. Dicks suggested the element of former companion Jo Grant returning the Metebelis Three crystal she had been given by the Doctor in The Green Death. The script also looked to the future: in Episode Two, the Brigadier originally referred to the UNIT medical officer as “Sweetman”, but this was amended to ”Sullivan” to presage the introduction of new companion Harry Sullivan in the subsequent serial, Robot.

Letts' agreement with the BBC gave him the option to direct one story every recording block. Having last handled 1973's Carnival Of Monsters, he now elected to direct Planet Of The Spiders himself. Letts thus fulfilled a longstanding ambition to direct a script he had helped write. To make his lead actor's final serial a special occasion, Letts assembled a cast which largely consisted of Pertwee's friends, together with guest performers from earlier Doctor Who stories with whom the series star had enjoyed a good relationship. This included Kismet Delgado, Roger Delgado's widow, as one of the Spider voices.

Barry Letts felt that Ian Scoones' five-foot prop for the Great One was too frightening

In pre-production, considerable effort was invested by the BBC Visual Effects Department in assembling the various arachnid props which were required for Planet Of The Spiders. For the Great One, assistant Ian Scoones designed a five-foot spider with red eyes which appeared to breathe by means of an inflatable bladder. Letts felt that this creation was too frightening, and instead opted to use the same prop which had been designed for the Spider Queen.

Meanwhile, Letts and Dicks spent the early part of 1974 engaged in a hasty search for an actor to play the Fourth Doctor. Also involved was Bill Slater, the BBC's incoming Head of Serials. Seeking a contrast with Pertwee's action-oriented Doctor, they felt that the new incarnation should be young in spirit but elderly of frame. As such, they considered a variety of actors who could be convincingly presented as older men. These included Ron Moody, who had played Fagin in the 1968 film Oliver! and had previously been approached in 1969 to succeed Patrick Troughton as the Doctor; Jim Dale, a regular actor in the Carry On series of comedy movies, who was interested but unavailable; Richard Hearne, who thought Letts wanted him to play the Doctor in the vein of the Mr Pastry character he had been portraying since 1949's Helter Skelter; Michael Bentine of The Goon Show, who withdrew when he learned that he would not have script input; Graham Crowden (later Soldeed in 1980's The Horns Of Nimon), who did not want to make a long-term commitment to the series; Bernard Cribbins, who had appeared in the 1966 film Daleks: Invasion Earth 2150 AD and would later play the recurring character of Wilfred Mott, but whose ideas for the Fourth Doctor were at odds with the production team; and, apparently, Tommy Steele, a former pop star turned actor.

Letts and Dicks were seriously considering offering the role to Fulton Mackay (who had played Dr Quinn in 1970's The Silurians) when Slater received a letter on February 5th from an actor named Tom Baker, with whom he had worked on a 1972 edition of Play Of The Month. Baker had recently found himself out of work, making ends meet as a construction labourer, and was writing with some desperation in the hope that Slater might be able to offer him a job. Thinking that the eccentric Baker might be suitable to play the new Doctor, Slater encouraged Letts and Dicks to watch his performance in the film The Golden Voyage Of Sinbad, which was then in theatres. Letts was so impressed with Baker that he discarded the “older Doctor” idea on the spot. Following a meeting on February 7th, Letts immediately offered an astonished Baker the starring role in Doctor Who. Baker was unveiled to the press on February 15th; he was formally contracted for twenty-six episodes on the 19th.

Location filming for Planet Of The Spiders began on March 11th in Berkshire: Mortimer Railway Station at Stratfield Mortimer was the train station where Mike collected Sarah, Tidmarsh Lane at Tidmarsh was the venue for the rural road scenes (for which the illusory tractor replaced a scripted cow), and nearby Tidmarsh Manor was K'anpo's monastery. The remainder of the filming schedule comprised material for Episode Two's lengthy chase sequence. March 12th took Letts' team to Membury Airfield in Membury, Wiltshire for aerial shots and scenes involving the gyrocopter. On this day, John Dearth -- who played Lupton -- barely avoided injury when the gyrocopter came apart after escaping its moorings, and part of a rotor blade sliced into his jacket. More material at Membury was filmed on the 13th, for which Letts provided the voice on the police radio. Cast and crew then travelled to the Le Marchant Barracks in Devizes, Wiltshire, which served as UNIT headquarters. Finally, March 14th and 15th went spent on the River Severn near Westbury, Gloucestershire, for the speedboat and hovercraft segment of the pursuit.

Planet Of The Spiders followed the usual Doctor Who studio recording pattern and was taped in fortnightly two-day blocks, this time on Tuesdays and Wednesdays. The first session occurred on April 2nd and 3rd at BBC Television Centre Studio 1 in White City, London. The Tuesday saw the completion of Episode One, as well as scenes set in the Doctor's laboratory for Episodes Three and Six. The latter included the regeneration scene, and marked Tom Baker's first recording for Doctor Who. Nicholas Courtney ad-libbed the Brigadier's closing line: “Well, here we go again.” Episode Two was then recorded on the Wednesday.

The second block, on April 16th and 17th in TC8, was devoted solely to Earthbound sequences from the final four installments, with those from Episodes Three and Four recorded on the Tuesday, and those from Episodes Five and Six on the Wednesday. As such, the Metebelis Three scenes were the focus of the final session, on April 30th and May 1st in TC6. The Tuesday dealt with material in the town square, the hut, the Spider Queen's chambers, and at the castle gates. This left sequences in the Cave of Crystal, the cell, the council chambers and the corridors for the Wednesday.

After an overrun on May 1st, the studio electricians turned off the power, preventing Barry Letts from finishing his shots

Unfortunately, delays had dogged the shoot -- largely stemming from complex effects requirements -- and caused frequent overruns. After recording on May 1st had already continued half an hour past its allotted time, the studio electricians turned off the power, preventing an angry Letts from finishing a small number of shots. Although this completed production on Season Eleven, the recording block would continue with Tom Baker's first story, Robot; it would be held over to start Season Twelve.

In editing, Letts found that Episode Three was too short. The cliffhanger was originally the Doctor's emergence from the TARDIS to be confronted by the guards, but material from Episode Four, up to the Doctor being shot, was pulled forward. However, this and other cuts then caused Episode Four to underrun. Again, a new cliffhanger was created by pilfering the intended opening of Episode Five: the credits would now roll after Sarah's realisation that the Doctor was also a prisoner, rather than after Lupton's command to kill the Doctor.

This, in turn, left Episode Five -- which originally concluded with the Doctor and Sarah being surrounded in the monastery cellar -- short by more than three minutes. To make matters worse, the early part of Episode Six did not contain a moment worthy of a cliffhanger. As such, Letts was forced to bring forward the scene of the attack on Tommy, with the awkward consequence that Episode Five's climax wasn't reprised until several minutes into Episode Six. Letts also compensated for the loss of material from the concluding installment by adding a flashback to the Cave of Crystal sequence.

The broadcast of Planet Of The Spiders saw Doctor Who's usual 5.30pm timeslot disrupted, with episodes variously scheduled up to fifteen minutes later. With Episode One on May 4th, The Wonderful World of Disney moved onto the Saturday evening schedule after Doctor Who, replacing Clunk-Click. The programme's lead-in remained the combination of Grandstand, a Walt Disney animated short, and a news update, although the cartoon was dropped prior to both Episodes Three and Four. The latter also saw a documentary about the Windsor Safari Park, hosted by Michael Aspel, aired prior to The Wonderful World Of Disney.

On June 8th, Jon Pertwee's successful run as the Third Doctor came to an end with the broadcast of Planet Of The Spiders Episode Six. This was also the last regular involvement in the programme for both Richard Franklin and Robert Sloman, and the last time that Terrance Dicks would be credited as script editor -- although he had already convinced his successor, Robert Holmes, to commission him to write the Fourth Doctor's debut serial. Pertwee had been inarguably essential to the revitalisation of Doctor Who for the Seventies, broadening the programme's appeal well beyond that of a traditional children's serial. But, as his features blurred into those of Tom Baker, few viewers could have realised that they were looking at the man who would take the show to unprecedented heights...

• Doctor Who Magazine #314, 6th March 2002, “Archive: Planet Of The Spiders” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who Magazine Special Edition #2, 5th September 2002, “The Show Must Go On” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #21, 2017, “Story 74: Planet Of The Spiders”, edited by John Ainsworth, Hachette Partworks Ltd.
• Doctor Who: The Handbook: The Third Doctor by David J Howe and Stephen James Walker (1996), Virgin Publishing.
• Doctor Who: The Seventies by David J Howe, Mark Stammers and Stephen James Walker (1994), Virgin Publishing.