A manned mission to the Red Planet, Mars Probe 7, is returning to Earth, but there has been no communication from the astronauts for several months. Recovery 7 meets it in orbit, but then the signal is suddenly cut off by an alien noise. When the recovery capsule touches down, UNIT is ambushed and the occupants vanish. However, the Doctor and Liz discover that the capsule interior is highly radioactive: whatever came down in Recovery 7 couldn't possibly have been human. As the Doctor searches for the missing astronauts, he begins to realise that he is up against a vast conspiracy reaching into outer space.

After completing work on the scripts for The Wheel In Space in early 1968, David Whitaker was approached by Doctor Who story editor Derrick Sherwin about contributing again to the programme. Set in the present day, it would depict humanity's first encounter with alien life. The storyline -- apparently called “The Invaders From Mars” -- continued to be developed into 1969, by which time Terrance Dicks had become script editor (as the post had been renamed) following Sherwin's promotion to producer.

On May 1st, Whitaker was commissioned to write a six-part storyline under the title of “The Carriers Of Death”. He had to account for a number of forthcoming changes to Doctor Who's format: a new incarnation of the Doctor would be exiled to Earth, and companions Jamie and Zoe would be replaced by Liz Shaw and the United Nations Intelligence Taskforce (UNIT). At this stage, Reegan was the principal villain, while Heldorf was a German scientist named Kuhn. Full scripts were requested on June 25th, with the serial extended to seven episodes. At this stage, “The Carriers Of Death” was scheduled to be the second story of Doctor Who's seventh season.

Derrick Sherwin decided that David Whitaker's approach was not what he wanted

Unfortunately, problems with the serial soon arose. After Whitaker turned in his first two scripts during July, Sherwin decided that the writer's approach was not what he wanted. Assistant script editor Trevor Ray was given the task of overhauling Episode One, and his revised version was sent to Whitaker on August 11th to provide an idea of what the production team wanted. A meeting was then held on the 13th to discuss the first two installments and the in-progress Episode Three. Work proceeded haltingly over the coming months, however, with Whitaker and the production team unable to come to terms with a suitable way of moving forward.

Finally, after the delivery of Episode Three on November 12th, it was agreed that Whitaker would cease further work on the serial. Instead, on November 18th, Malcolm Hulke -- who had recently completed The Silurians, the serial intended to follow “The Carriers Of Death” -- was commissioned to revise Episodes Two and Three, and write the final four installments himself. Hulke would have free reign to alter the storyline where necessary, but Whitaker would maintain sole televised credit on all seven parts. Sadly, this marked an ignominious end to the involvement in Doctor Who of its original story editor. To allow Hulke time to complete his task, it was decided to transpose The Silurians and “The Carriers Of Death” in the running order.

Various names changed as the adventure underwent its difficult development: Dawson became Dobson and Cunningham became Carrington, while Taltalian's name was originally spelt “Taltalien”. The two legends on Reegan's van initially referred to Progressive Launderers Ltd and Mason's Bakery, but these were later altered to Hayhoe Launderers Ltd and Silcock Bakeries in honour of assistant floor manager Margot Hayhoe and director's assistant Pauline Silcock. Additional rewrites were carried out by Dicks and the assigned director, Michael Ferguson, who had most recently handled The Seeds Of Death late in the preceding season. Meanwhile, although new producer Barry Letts was not entirely satisfied with the character of Liz Shaw, actress Caroline John was contracted for the final two serials of Season Seven on January 5th, 1970.

On January 23rd, filming for “The Carriers Of Death” got under way in Little Marlow, Buckinghamshire. The day's work began at the Wycombe RDC Sewage Purification Works, which served as the isotope factory. In the afternoon, Reegan dumped his henchmen's bodies at Folley's Gravel Pit. After the weekend, work resumed with two days in Southall, London -- January 26th and 27th -- at the Southall Gas Works, which served as Space Headquarters. Part of the 27th was also spent recording warehouse footage on nearby White Street, and the remainder of these sequences was completed at the former TCC Condensers in Acton, London on the 27th and 28th.

Ferguson's team returned to Buckinghamshire on January 29th. Wycombe Air Park in High Wycombe served as Heldorf's lab, while the scenes of Liz being pursued were filmed in Marlow, on Gossmore Lane and at the Marlow Weir. Caroline John was forced to wear a blonde wig, as her hair frizzed badly in the rain. She was also privately very nervous, having just discovered that she was pregnant with her first child. Cast and crew next relocated to Aldershot, Hampshire on January 30th and 31st. On the first day, the retrieval of Recovery 7 was filmed at Claycart Bottom, Puckeridge Hill Road and Fleet Road. The next day, the capsule's hijack was staged at the Royal Engineers Driving Circuit at Farnborough Airport. Unfortunately, Pauline Silcock suffered a leg injury when a riderless motorcycle crashed into the camera crew. Work was also delayed due to a reported UFO sighting -- until it was realised that the alleged UFO was the Recovery 7 prop itself.

Following a day off, material outside Reegan's lair was shot at Beacon Hill, Hampshire on February 2nd. Finally, on the 3rd and 4th, scenes at the Space Headquarters main gate was captured at Blue Circle Cement Works in Northfleet, Kent. Amusingly, the facility was in the midst of an industrial dispute, but the goodwill generated by the presence of the Doctor Who team brought an end to the strike -- prompting the owners to waive the fee for the use of their premises! Around this time, the serial's title became The Ambassadors Of Death, with some consideration apparently also given to the simpler “The Ambassadors”.

Model filming took place at the BBC Television Film Studios in Ealing, London from February 2nd to 4th. Studio taping then began on Friday, February 13th. Unlike The Silurians, on which problems with sets had prompted Barry Letts to experiment with the recording pattern, The Ambassadors Of Death employed the traditional production schedule of completing one episode per week. The first five installments of The Ambassadors Of Death were taped at BBC Television Centre Studio 3 in White City, London. Unusually, to save on costs, the Recovery 7 set was a co-financed venture between the Doctor Who production office and the BBC drama series Doomwatch, for which it served as Sunfire One in the March 1970 episode Re-Entry Forbidden.

Caroline John's husband, Geoffrey Beevers, joined the cast for Episode Seven as Private Johnson

For Episode Five, recorded on March 13th, the production team decided to replace the role of the scripted sergeant (whose surname was apparently West) with John Levene as Sergeant Benton. Benton had been introduced as a corporal in the previous season's The Invasion -- the story which had introduced the UNIT concept -- and had already been invited back for Inferno, the next story in production. Recording shifted to TC4 for Episode Six before moving to TC1 for the last installment, taped on March 27th. Caroline John's husband, Geoffrey Beevers, joined the cast for this episode as Private Johnson; her brother, Nicholas John, was also working on the serial as a production assistant.

The broadcast of Episode Three on April 4th saw a small change to the schedule, with Doctor Who pushed back by five minutes to 5.20pm to accommodate the broadcast of the Grand National. A bigger disruption occurred the following week, when Apollo 13 was scheduled to blast off from Cape Kennedy, Florida. As a result, The Debbie Reynolds Show -- which normally followed Doctor Who -- was preempted, and The Ambassadors Of Death Episode Four was instead scheduled to be followed by a repeat of Dad's Army, which usually aired after the American sitcom. However, April 11th was also the date of the FA Cup Final between Chelsea and Leeds United. Because it went into extra time (and still ended in a draw), the start of Doctor Who was delayed by half an hour. As such, it led directly into a news update and then the 1953 Rita Hayworth film Salome, which aired as part of the BBC's High Adventure strand.

• Doctor Who Magazine #252, 4th June 1997, “Archive: The Ambassadors Of Death” by Andrew Pixley, Panini UK Ltd.
• Doctor Who Magazine Special Edition #2, 5th September 2002, “Instant Karma” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #15, 2016, “Story 53: The Ambassadors Of Death”, 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.