When a manned mission to Mars returns to Earth, it soon becomes apparent that the three beings who disembark are not the ship's astronauts. The Doctor realises that the crew have made contact with an alien force on the Red Planet, but his investigations are interrupted when the aliens masquerading as the astronauts are kidnapped by someone who knows them of old.

After completing work on his Season Five adventure The Enemy Of The World, original Doctor Who story editor David Whitaker was approached in 1968 by then-story editor Derrick Sherwin about writing a new six-part serial concerning humanity's first present-day 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 Doctor Who's script editor (as the post had been renamed) while Sherwin had been promoted to producer.

On May 1st, Whitaker was commissioned to write a seven-episode storyline under the title of “The Carriers Of Death”. By now, however, it was known that the series' format would be undergoing a radical redesign in the coming year; the Doctor would be regenerated and exiled to Earth, and companions Jamie and Zoe would be replaced by Liz Shaw and UNIT, the United Nations Intelligence Taskforce. In putting together his storyline, then, Whitaker was asked to revise his original ideas for the adventure in order to account for these changes. Full scripts were requested on June 25th; “The Carriers Of Death” was at this point scheduled to be the second story of Season Seven.

Problems quickly developed with the serial, however. 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 revising episode one, and this was sent to Whitaker on August 11th to give him 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 part 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, in mid-November, it was agreed that Whitaker would deliver nothing past the third episode. Instead, Malcolm Hulke -- who had recently completed The Silurians, the serial intended to succeed “The Carriers Of Death” -- was commissioned on November 18th to revise the delivered parts two and three, and write the final four installments himself, making changes to Whitaker's storyline where necessary. Whitaker, however, would maintain sole televised credit on the serial. Furthermore, to allow time for Hulke to complete his task, it was decided to transpose The Silurians and “The Carriers Of Death” in the running order; the latter therefore became Serial CCC.

Amongst the changes made to the adventure between Whitaker's original storyline and Hulke's final drafts was the removal of an Army character named Lieutenant Pollard from episodes two and three. As well, Professor Heldorf was initially a German scientist named Kuhn, while Dobson was called Dawson, Carrington was initially Cunningham, and Taltalian's name was spelt “Taltalien”. As well, the two names on Reegan's van were originally to have been Progressive Launderers Ltd and Masons Bakery, but these were later changed to Hayhoe Launderers Ltd and Silcock Bakeries in honour of assistant floor manager Margot Hayhoe and director's assistant Pauline Silcock. Additional rewriting on “The Carriers Of Death” was performed by Dicks and Michael Ferguson. Ferguson had been assigned as the story's director, having most recently handled The Seeds Of Death some months earlier.

Although Letts was not entirely satisfied with the character of Liz, Caroline John was contracted for the final two serials of Season Seven on January 5th, 1970. Filming finally got under way on January 23rd. Two Buckinghamshire locations were used: the Little Marlow Sewage Treatment Works was the isotope factory, while the gravel pit was Folley's Gravel Pit at Spade Oak. After the weekend, worked resumed with two days at the Southall Gas Works in Middlesex, which served as Space Headquarters. Some warehouse footage was shot on January 27th on nearby White Street, and the remainder was completed at TCC Condensers in Ealing on the 27th and 28th.

Buckinghamshire was again the location of choice on the 29th. Wycombe Air Park in High Wycombe served as Heldorf's lab, while the scenes of Liz being pursued were filmed on roads in Marlow and at the Marlow Weir. John was forced to wear a blonde wig in these sequences as her hair frizzed badly in the rain. The retrieval of Recovery 7 was enacted the next day at Aldershot, Hampshire on January 30th and 31st. On February 2nd, nearby Beacon Hill was the site of Reegan's lair. Finally, on the 3rd and 4th, more Space Headquarters footage was captured at Blue Circle Cement Works in Northfleet, Kent.

At about this time, the serial's title became The Ambassadors Of Death. Studio taping then began on Friday, February 13th. Unlike The Silurians, on which new producer Barry Letts had experimented with the recording pattern, The Ambassadors Of Death employed the traditional studio schedule of completing one episode every seven days. The first five installments were all taped in BBC Television Centre Studio 3. Unusually, to save on costs, the Recovery 7 set was a cofinanced venture between the Doctor Who production office and the BBC drama series Doomwatch, for which it served as Sunfire One in the episode Re-Entry Forbidden.

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 part six before moving to TC1 for the last installment, taped on March 27th. Caroline John's husband, Geoffrey Beevers, joined the serial for this episode, playing Private Johnson.

The Ambassadors Of Death was Whitaker's last, and unfortunately least favourite, Doctor Who story. It was also amongst his final BBC work, after which he continued writing, including a stint in Australia. Whitaker took ill shortly after agreeing to novelise The Enemy Of The World for Target Books. Sadly, it was to be a project Whitaker would never see through to completion, as he died of cancer on February 4th, 1980.

• 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 #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.