After being attacked in space, the TARDIS materialises on Earth. There the Doctor is reunited with his old friend, Lethbridge-Stewart, who is now a brigadier in charge of the newly-formed United Nations Intelligence Taskforce: UNIT. The Doctor and Jamie are recruited to help UNIT's investigation of a company called International Electromatics, run by the sinister Tobias Vaughn, while Zoe meets Isobel Watkins, whose scientist father is missing. Vaughn is facilitating an invasion of the Earth, while forcing the captive Watkins to construct a device to double-cross his allies -- the Cybermen!

By early 1968, Kit Pedler had developed four Doctor Who storylines featuring his co-creations, the Cybermen. The most recent of these, The Wheel In Space, was still in pre-production when producer Peter Bryant asked for another story featuring the silver giants. Pedler duly conceived a six-part adventure, apparently under the title “Return Of The Cybermen”. Since Pedler was not a professional, the past practice had been to hire another writer to turn the storyline into full scripts. However, neither of Pedler's previous collaborators -- David Whitaker and Gerry Davis -- were available. On the other hand, current story editor Derrick Sherwin was expecting to be moved off Doctor Who to become a producer on a series of plays. As such, it was agreed that Sherwin would write the Cyberman serial while his assistant, Terrance Dicks, would be promoted to script editor (as the position had now been renamed).

Sherwin and Pedler's adventure was intended to be the first of Doctor Who's sixth production block. However, it would follow The Dominators and The Mind Robber in the running order, both of which were being held over from the previous set of episodes. Bryant felt that there was not a lot of usable content in Pedler's storyline, and indicated that Sherwin should just use it as a springboard. However, any notion of abbreviating the commission was quickly abandoned when problems became evident on subsequent serials -- including Paul Wheeler's “The Dreamspinner”, which was intended to follow it into production. As such, the Cyberman story would instead be expanded to eight episodes, making it the longest Doctor Who adventure since the twelve-part The Daleks' Master Plan in 1965.

The title of The Invasion was intended to keep the appearance of the Cybermen a secret

Sherwin was commissioned to write The Invasion on May 6th; the new title was intended to keep the appearance of the Cybermen a secret. When Sherwin's other project fell through, he was later granted permission for a staff contribution on July 8th. With the success of Season Five's The Web Of Fear, Bryant and Sherwin both felt that Doctor Who would be better served if its focus was directed away from stories set in the far future or on alien worlds, and was instead concentrated on adventures which took place on Earth, in the modern day or the near future. The production team thought that The Invasion could be used as a pilot of sorts for this new vision; should it be successful, Doctor Who could then be appropriately retooled for its seventh season. To this end, Douglas Camfield, who had directed The Web Of Fear, was assigned to The Invasion.

Initially, it was thought that The Invasion might see the return of Driver Evans, Professor Travers and Colonel Lethbridge-Stewart, all of whom had appeared in The Web Of Fear. Evans was then quickly ruled out, due to fears that such a broad Welsh stereotype would be inappropriate for a recurring role. In early May, Camfield sought permission for the characters' use from their creators, Mervyn Haisman and Henry Lincoln. By the end of the month, however, it was determined that the role for Travers was not significant enough to merit the expense of hiring Jack Watling to reprise his role, and the character of Professor Watkins was duly created as a replacement. (Similarly, Isobel Watkins may have been a substitute for Travers' daughter, Anne.)

Given the potential reuse in later stories of elements established in The Invasion, Bryant also considered the possibility of replacing Lethbridge-Stewart with a character which would be wholly BBC-owned. However, assistant head of copyright John Henderson noted that such a character would be viewed as legally interchangeable with Lethbridge-Stewart, and hence would still be deemed the property of Haisman and Lincoln. Meanwhile, it was decided that Lethbridge-Stewart should be promoted to brigadier for The Invasion, and that the character would now be the head of an investigative military organisation. It appears that it was Camfield who coined the name for this group: the United Nations Intelligence Taskforce or UNIT.

At an early stage, it was planned that the Cybermats -- the silverfish-like cyborgs which had been seen in the last two Cyberman serials -- would be included in The Invasion. Also dropped was an appearance by the Servo Robot from The Wheel In Space, which would have emerged from within the Cyber Director to confront Tobias Vaughn. One scene which was intended to be filmed was the rescue of Professor Watkins in Episode Six. Here, UNIT trapped the International Electromatics vehicle carrying the scientist, who emerged and ran towards the soldiers. Benton shot Gregory when the IE man levelled his gun at the fleeing Watkins.

Frazer Hines informed Peter Bryant that he intended to leave Doctor Who after two more stories

On April 30th, Frazer Hines had been contracted for The Invasion and the serial which followed it; following the abandonment of “The Dreamspinner” on April 9th, this was now Dick Sharples' “The Prison In Space ”. Hines informed Bryant that he intended to leave Doctor Who after these two stories, as he and his agent felt that it was time for Hines to take advantage of the popularity he had accrued during his two years on the show. As such, Hines' departure was announced to the press on September 5th.

Around the middle of August, however, Patrick Troughton also decided that Season Six would be his last on Doctor Who, and he told Bryant that he would remain on the show until the following spring. By the end of the previous recording block, Troughton had become extremely fatigued, and he was also growing increasingly worried that he would be typecast as the Doctor. Troughton then asked Hines to prolong his stay on Doctor Who so that the two actors could finish up together. Hines' father had recently passed away and he was consequently more reticent to give up his Doctor Who salary. Against his agent's advice, Hines agreed to Troughton's plan.

Pre-production on The Invasion continued throughout the summer. A significant contribution was made by costume designer Bobi Bartlett, who opted to introduce a new appearance for the Cybermen. The most substantial change was to the creatures' helmets, with Bartlett building up the left and right sides in an “earmuff” motif. The Cyber Director was a modified version of the prop used for the Cyber Planner in The Wheel In Space. Camfield, meanwhile, was successful in securing considerable involvement from the Ministry of Defence, who appreciated the positive light in which Sherwin's script portrayed the military. The Ministry promised the use of facilities, vehicles, equipment and troops for the serial.

Amongst the guest artistes for The Invasion was John Levene, who was intended to play a Cyberman, as he had on The Moonbase. For Camfield, Levene had previously been a Yeti in The Web Of Fear. Unexpectedly, however, he found himself given the opportunity to tackle a meatier role when the actor originally cast as Corporal Benton was dismissed for chronic tardiness, and Camfield reallocated the part to Levene. Camfield also cast his wife, Sheila Dunn, as both the International Electromatics computer voice and the telephone operator. Playing Isobel was Sally Faulkner, a young actress whose few credited roles to date included episodes of Boy Meets Girl and the BBC Play Of The Month. Bryant and Sherwin were considering bringing Isobel back as a regular character if the potential new format for Season Seven was implemented, and so Faulker would receive significant attention as work on The Invasion got under way.

Doctor Who's sixth production block saw a significant change in the way recording was scheduled. Up to this point, pre-filming had occurred during rehearsals for other serials, enabling the completion of more than forty episodes per year. In light of Troughton's exhaustion, however, it had now been agreed that this overlap would be eliminated, giving each story a dedicated week for pre-filming -- or two weeks, in the case of a story with the length of The Invasion. The trade-off was that the new production block would see the completion of only thirty-four installments.

The lengthy location shoot began on September 3rd in Gloucestershire

Recording for The Invasion began on August 30th with model work at the BBC Television Film Studios in Ealing, London. The lengthy location shoot then began, with the first three days all spent at venues in Gloucestershire. Things got off to a faltering start on September 3rd when the helicopter intended for use at RAF Fairford proved unavailable due to insurance problems. Nonetheless, scenes outside UNIT's transport plane were recorded there, before cast and crew moved to Williamstrip Farm at Coln St Aldwyns, which provided the field where the TARDIS landed. The next day was also spent at Coln St Aldwyns, with Hatherop Road serving as the lane where the time travellers encountered the lorry driver. Most of the deferred helicopter sequences were rescheduled for September 5th, when Kingston Minerals in Kempsford also stood in for the International Electromatics compound. On September 6th, scenes on the roof of the IE offices were taped atop the Associated British Malsters factory in Wallingford, Oxfordshire, during which the remaining helicopter material was completed.

The rest of the location shoot took place in London, starting on September 7th when the Millbank Tower in Westminster was the exterior of IE's London offices. September 8th was a busy day, during which Camfield's team moved around to several venues, mostly in the City of London proper. Moor Lane and Fore Street were where Jamie, Zoe and Isobel accessed the sewers. Establishing shots of London immediately prior to the Cyberman invasion were captured at Cumberland Terrace near Regent's Park in Camden, St Paul's Churchyard, and Australia House. Londoners collapsed at Distaff Lane and Queen Victoria Street. The Cybermen advanced along Knightrider Street and -- most memorably -- down St Peter's Hill in front of St Paul's Cathedral. Camfield had also hoped to film the Cybermen marching at a number of other London landmarks -- including Tower Bridge, the Houses of Parliament and Hyde Park -- and emerging from the entrances to several tube stations, but he ran out of time before the Sunday crowds became unwieldy.

September 9th and 10th were dedicated to the battle between UNIT and the Cybermen at the International Electromatics factory, which was staged at TCC Condensers in Acton, London. The 2nd Battalion of the Coldstream Guards was on hand to play UNIT troops. September 11th was another day that saw several locations in use. On Princedale Road in Notting Hill, Camfield himself played the driver who deposited the time travellers near Travers' home. Elsewhere in Notting Hill, the Doctor and Jamie were followed by UNIT along Walmer Road, and then were cornered on Heathfield Street. The exterior of Travers' house was a residence at St James' Gardens in Kensington. Finally, more IE compound material was completed at the Guinness Brewery in Acton, as were sequences in the goods yard. Patrick Troughton and Frazer Hines took full advantage of the brewery's lunchtime hospitality, and soon found themselves struggling not to fall over during the afternoon recording...

More footage at TCC Condensers was completed on September 12th. The last day of location filming was the 13th, when the scene of the Doctor and Jamie in the canoe was recorded in Regent's Canal at Lisson Grove in Westminster. Part of the day was also spent back at the BBC Television Film Studios to capture the material in the lift shaft. Unfortunately, despite the length of the location shoot, Camfield was unable to complete the action scene in which UNIT recaptured Professor Watkins. As such, the script for Episode Six was rewritten to have Packer report the events verbally to Vaughn, while Gregory would now be killed by the Cybermen in the sewers.

Although several episodes near the end of the fifth production block had enjoyed recording at BBC Television Centre, Doctor Who now returned to Lime Grove Studio D in Shepherd's Bush, London. All eight episodes of The Invasion were recorded there, on consecutive Fridays from September 20th. For Episode Two, taped on the 27th, the photos of the missing included Camfield, Sherwin, Bryant and Dicks. On October 4th, Wendy Padbury enjoyed a week's holiday, as Zoe did not appear in Episode Three. Unusually, Episodes Five and Eight were recorded largely out of sequence, with Camfield opting to group together all of the scenes on a given set before moving on to the next; this was becoming an increasingly common technique in the industry. For the concluding installment, taped on November 8th, it was Hines' turn to get a week off, since Jamie only appeared in the final scene, which had been filmed on location.

During recording, Bryant approached Courtney about returning to Doctor Who on a regular basis the following year; the actor agreed without hesitation. Deeming The Invasion a success, Bryant and Sherwin were now moving ahead with plans for a very different Season Seven -- one in which the Brigadier and the UNIT organisation would feature heavily.

At one stage, it was expected that The Invasion would air from October 19th to December 7th, and that Doctor Who would then go on hiatus for the final three weeks of 1968. These plans changed around early September, however, when it was decided that Grandstand -- which immediately preceded Doctor Who -- would be expanded for two weeks to provide coverage of the Olympic Games from Mexico City, Mexico. As a result, Doctor Who was preempted on both October 19th and 26th, before The Invasion began transmission on November 2nd. The December break had been jettisoned, with the next story -- now The Krotons -- following on directly from the broadcast of The Invasion Episode Eight, on December 21st.

• Doctor Who Magazine #189, 5th August 1992, “Archive: The Invasion” by Andrew Pixley, Marvel Comics UK Ltd.
• Doctor Who Magazine Special Edition #4, 4th June 2003, “Paradise Lost” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #13, 2015, “Story 46: The Invasion”, edited by John Ainsworth, Hachette Partworks Ltd.
• Doctor Who: The Handbook: The Second Doctor by David J Howe, Mark Stammers and Stephen James Walker (1997), Virgin Publishing.
• Doctor Who: The Sixties by David J Howe, Mark Stammers and Stephen James Walker (1992), Virgin Publishing.