The Time Lords send the Doctor and Jo to an Earth colony in the 25th century. There, the time travellers discover the colonists being ravaged by a weird dinosaur-like beast while sinister miners try to force them to abandon the planet. Meanwhile, the Master has also arrived, searching for a legendary doomsday device believed to be buried in the ruins on the outskirts of the colony.

When Barry Letts became producer of Doctor Who in late 1969, he was generally in approval of the new format introduced by his predecessors, Peter Bryant and Derrick Sherwin, at the start of Season Seven. However, Letts did have some specific concerns, amongst them the perceived limitations of the Doctor's new Earthbound status. Letts was supported in this view by writer Malcolm Hulke, who had expressed concern that Doctor Who's new format left room only for stories about alien invasions and mad scientists, although the production team had worked hard during Season Seven to disprove this notion. Hulke himself had contributed The Silurians and had performed considerable rewriting on David Whitaker's The Ambassadors Of Death.

Nonetheless, Letts was determined that Doctor Who's eighth season would see the Doctor venture, in some fashion, back into outer space. On June 29th, 1970, Hulke was commissioned to pen a storyline titled “Colony”, which would take as its inspiration the days of the Wild West and the strife between settlers and Native Americans. Hulke was asked to incorporate the Master into “Colony”, but by this stage, Letts and script editor Terrance Dicks were aware that their plan to feature the new archvillain in all five Season Eight stories risked making the adventures seem predictable and boring. As such, it was agreed that Hulke would not bring the Master to the forefront until midway through the serial. “Colony” would also see a minimal presence from the UNIT characters, with Benton and Yates entirely absent and the Brigadier present only in the opening and closing installments.

The six scripts for “Colony” were commissioned on September 15th. At this stage, two character names were altered -- John Ashe's first name became Robert, while Reeves was changed to Winton -- and the serial itself became known as Colony In Space. The alien planet, originally unnamed, was christened Uxarieus, while the setting was shifted from around the year 3000 back to 2471. Running fourth in the season, Colony In Space was denoted Serial HHH.

The director assigned to the story was Michael Briant. A former child actor, Briant had risen through the ranks at the BBC, and had previously worked on Doctor Who in a number of capacities, including assistant floor manager and production assistant, the latter most recently on Fury From The Deep three years before. Since becoming a director, Briant had helmed episodes of series such as Z Cars.

It was Briant who made a major change to the story, by casting Susan Jameson in the role of Morgan, which Hulke had written for a man. However, Head of Drama Serials Ronnie Marsh overruled Briant, fearing that a female Morgan might impart unwanted fetishistic overtones. The part was thus given to Tony Caunter, who had initially been cast in a more minor role. Jameson was told of the decision on January 27th, 1971, although she was paid her full salary nonetheless. Another story change which came during preproduction was to make the IMC robot boxy and machinelike, Hulke having originally conceived of it as more humanoid in shape.

All the location filming for Colony In Space took place at the Old Baal China Clay Quarry in Carclaze, Cornwall, spanning February 10th to 16th, with the exception of the weekend (the 13th and 14th). The shoot was made difficult by cold, driving rain and resulting mud. Amongst other difficulties, the inclement weather caused the destruction of the garden planted outside the colonists' dome.

All three studio sessions for Colony In Space were recorded fortnightly on Fridays and Saturdays, beginning on March 5th. The first of these took place in Television Centre Studio 4, while the remaining two blocks shifted to TC3. During rehearsals for the first studio session, Jon Pertwee was the subject of an episode of This Is Your Life. It was recorded on March 3rd, with Pertwee ambushed by host Eamonn Andrews when Letts lured him and Katy Manning out to a BBC parking lot, ostensibly to perform reshoots for some of the location scenes. The This Is Your Life episode later aired on April 14th.

Each studio night for Colony In Space was generally devoted to completing the scenes for one episode. An exception was the part three material set at the Leesons' dome; this was taped on March 6th, which was otherwise chiefly devoted to episode two. The final episode, recorded on April 3rd, saw Briant's wife Monique make a cameo appearance as one of the colonists.

• 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 #238, 8th May 1996, “Archive: Colony In Space” 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.