A spaceship captained by Gordon Lowery has landed on Kembel. One of his fellow astronauts, Jeff Garvey, becomes infected by a Varga plant, and is consumed with homicidal rage even as he starts to turn into a Varga plant himself. Garvey is killed by the ship's other passenger, Marc Cory, who reveals that he is an agent of the Space Security Service. His mission is to expose a Dalek plot being hatched at a secret base on Kembel -- one that involves a conspiracy spanning the stars. But with Daleks bearing down on him, and Lowery also succumbing to the Varga infection, can Cory survive to warn the galaxy?

By the end of 1964, twenty-six episodes had been confirmed for Doctor Who's second recording block. With the programme having settled into a pattern of four- and six-part stories, it was expected that the block would be extended by another eight episodes. Most of these would form part of the programme's second broadcast season, along with two serials which had been carried over from the first production block: Planet Of Giants and The Dalek Invasion Of Earth. In October, BBC Head of Serials Donald Wilson had ordered that Planet Of Giants be truncated from four episodes to three, due to a perceived lack of dramatic impetus. For a time, it was thought that the Dalek story planned to be made at the end of the second recording block (eventually The Chase) might be expanded from six episodes to seven as compensation, but nothing came of this. Nonetheless, it was likely for this reason that, in early 1965, Head of Drama Sydney Newman extended the second production block to thirty-five episodes, rather than the anticipated thirty-four.

At around the same time, writer Terry Nation was completing work on The Chase. Keen to repeat the success of The Dalek Invasion Of Earth, which aired in the run-up to Christmas 1964, producer Verity Lambert and story editor Dennis Spooner asked Nation to contribute another six-part Dalek story for broadcast around November and December 1965. It was decided that the recording block's extra episode would be used as a special “trailer” to build even more anticipation for the return of the Daleks. To save the cost of contracting the regular cast for the additional installment, it would be the first Doctor Who episode to feature neither the Doctor nor any of his companions.

Terry Nation considered spinning the Daleks off into their own show

Nation was commissioned to write this one-off, given the title “Dalek Cut-Away” or “Dalek Cutaway”, on February 25th. By this stage, the popularity of the Daleks was so great that Nation was considering the possibility of spinning them off into their own show -- perhaps even one which could be exported to the United States, a market which Doctor Who had so far failed to crack. He therefore viewed “Dalek Cutaway” as an opportunity to lay the groundwork for such a programme. To this end, Nation developed the concept of the Space Security Service (also called the Special Security Service and the Space Special Security Service). It would be represented by Agent Marc Cory, whom Nation conceived as a “space-age James Bond”, the British spy character having become an enormous hit in the wake of the 1964 film Goldfinger.

“Dalek Cutaway” was originally set on the planet Varga. Around July, the name was altered to Kemble and then Kembel. The Varga plants themselves retained their original name, but now became artificial creations of the Daleks, brought to Kembel from Skaro, as opposed to an indigenous form of life. The name Mission To The Unknown was applied to Nation's script, although “Dalek Cutaway” would continue to be used in some documentation. (Indeed, Mission To The Unknown is sometimes considered to be the title of the single episode which makes up a serial called “Dalek Cutaway”.) Rather than leading into a standard six-episode Dalek story, it was now known that Mission To The Unknown would serve as a prologue for an unprecedented twelve-part adventure: The Daleks' Master Plan.

Due to its unique nature, it was decided that Mission To The Unknown would be made by the same crew as the preceding serial, Galaxy 4, including director Mervyn Pinfield. The Dalek episode had previously been designated as Serial DC, but since Galaxy 4 was Serial T, Mission To The Unknown would now be variously referred to as Serial T/A, Serial Ta, and even “Serial T Episode 5”. Galaxy 4 was filmed at the BBC Television Film Studios in Ealing, London from June 22nd to 26th; Mission To The Unknown required only part of the last day, for Garvey's transformation into a Varga plant. By this time, Pinfield had fallen badly ill and was replaced at short notice by junior director Derek Martinus.

The recording of Mission To The Unknown formally concluded Verity Lambert's time as the producer of Doctor Who

With Martinus at the helm, the lone studio session for Mission To The Unknown was held on Friday, August 6th. Although it was originally planned to use Studio 3 at BBC Television Centre in White City, London, recording instead took place in Studio 4. The four principal Dalek casings retained by the BBC (two made for The Daleks, two for The Dalek Invasion Of Earth) were present. Work on this day brought Doctor Who's second production block to a close, and formally concluded Verity Lambert's time as the producer of Doctor Who.

Like Galaxy 4, Mission To The Unknown was held back to start Doctor Who's third season in the autumn of 1965. As such, it was broadcast on October 9th. At the same time, the BBC Saturday night schedule settled into a new pattern after being in flux for several weeks. Still preceded by Juke Box Jury and a news update, Doctor Who was now followed by the offbeat American sitcom The Munsters, which pushed Dixon Of Dock Green to later in the evening.

• Doctor Who Magazine #271, 18th November 1998, “Archive: Mission To The Unknown” by Andrew Pixley, Panini 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 #6, 2017, “Story 19: Mission To The Unknown”, 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.