UNIT is called in when a nuclear reactor on Wenley Moor starts experiencing strange power disruptions. The Doctor discovers that the activation of the reactor has accidentally awakened the Earth's original civilised inhabitants, the reptilian Silurians, who have lain in suspended animation underground for millennia. Now, the Silurians wish to reclaim their planet, and unleash a deadly virus which will engulf mankind.

Malcolm Hulke and Doctor Who script editor Terrance Dicks had collaborated on The War Games, the epic-length story which had concluded Patrick Troughton's tenure as the Doctor. Some months afterward, in the summer of 1969, Dicks approached Hulke seeking new story ideas for Jon Pertwee's Third Doctor. Hulke was initially reticent: producers Peter Bryant and Derrick Sherwin had reformatted Doctor Who such that it was now effectively Earthbound, and Hulke feared that this restricted the programme to adventures involving either alien invasions or mad scientists.

Nonetheless, Hulke decided to try to prove himself wrong, and developed an idea about a race of creatures who would be revealed as having inhabited the Earth millions of years before mankind. On June 15th, Dicks commissioned the six-part serial from Hulke under the title of “The Monsters”. The story gained both a less-generic name -- The Silurians -- and an extra episode around the time its scripts were greenlighted on August 11th. To this point, Hulke's adventure was intended to be the third serial of Season Seven, coming after David Whitaker's The Ambassadors Of Death. When that story hit severe scripting difficulties, however, Dicks elected to transpose the two in the running order, moving The Silurians up to second; it therefore received the production code Serial BBB.

The director assigned to The Silurians was Timothy Combe. Combe had been an assistant floor manager on The Keys Of Marinus and a production assistant on both The Reign Of Terror and The Evil Of The Daleks. He had only recently completed the BBC's internal directors' course, and had since helmed episodes of Z Cars and The Newcomers.

Meanwhile, major changes were afoot in the Doctor Who production office. Bryant and Sherwin had essentially been sharing the producer's duties in recent months, and although Sherwin had received the producer's credit on both The War Games and the first Pertwee story, Spearhead From Space, it appears that Bryant was intended to be the listed producer on The Silurians. In October, however, it was decided that both Bryant and Sherwin would be reassigned to a problematic thriller series called Paul Temple, and Barry Letts was named Doctor Who's new producer. However, Letts still had commitments to the soap opera The Doctors which would prohibit him from supervising the location filming for The Silurians; assistant script editor Trevor Ray therefore served as an associate producer in Letts' absence.

Production on The Silurians began a week late due to the same industrial action which had forced Spearhead From Space to be recorded entirely outside the studio. Consequently, filming began on November 12th at locations in Marylebone and Shepherd's Bush, London, for the sequence in episode six where the ill Masters arrives in the city. The next two days were spent at Sheephatch Farm in Tilford, Surrey, which served as Squire's farm. Publicity shots were also taken of Bessie, the Doctor's new car which would debut in The Silurians. Suggested to Bryant by both Pertwee and Ray, Bessie was built in the style of an Edwardian roadster and bore a fake license plate reading WHO1 (although since this plate number was already registered, its actual plate was the more mundane MTR5).

After a break for the weekend, work resumed on the 17th and 18th at Hankley Common in Rushmoor, Surrey, filling in for Wenley Moor. Wenley Hospital was actually Milford Chest Hospital in Milford, Surrey; scenes there were filmed on November 19th. Finally, two more Surrey locations -- the High Street in Godalming and the Hog's Back Transmitter Station in Guildford -- played host to several scenes on the 20th. Amongst these was the sequence where the Doctor watches UNIT blow up the Silurian caves, a scene apparently added by Dicks at Letts' request. Letts had discovered from audience research reports that more than half of Doctor Who's viewers were adults, and therefore sought to orient the programme in a more mature direction. Unfortunately, the filming of the explosion did not go as planned and a small grass fire was ignited, forcing Ray to explain the mishap to the local fire department.

Although this should have concluded filming for The Silurians, it was soon discovered that the processing lab had accidentally destroyed the footage from Marylebone Station. As a result, this material was remounted on November 24th. Several members of the production staff made cameo appearances, including Ray as the ticket collector and Letts and Dicks as passengers.

One location Combe had contemplated visiting for the serial was Wookey Hole in Somerset, in which he hoped to set all the cave scenes. However, it became clear that this would not be logistically feasible, and so Combe elected to move these sequences into the studio. Unfortunately, this lead to additional difficulties when designer Barry Newbery discovered that the outside contractors he had hired to build the cave set had made the walls too flimsy to stand erect. As a result, it was decided to abandon Doctor Who's usual pattern of taping all the scenes for one episode each week.

Instead, the first day -- Monday, December 8th -- saw the recording of all the material for part one not set in the caves. Several cave scenes were then taped on December 15th. Also recorded on this day were some of the opening credit sequences; unfortunately, in a break with tradition, the story's title was given as “Doctor Who and the Silurians”; although this was the style used on most Doctor Who scripts to this point, the “Doctor Who and...” prefix had always been omitted from the title given on-screen. To avoid a recurrence of this error in the future, the prefix was largely dropped from subsequent scripts.

Another innovation occurred the following week, which saw Doctor Who in studio on both Sunday, December 21st and Monday, December 22nd. Letts felt that the usual one-week-one-episode pattern was inefficient; instead, he thought that it might be better to tape two installments together on a biweekly basis. Since the sets could then be left up overnight, this would reduce the amount of damage they incurred from being taken down and put back up.

December 21st saw the completion of the remaining cave scenes. This included the first use of Colour Separation Overlay (CSO, also known as ChromaKey or Greenscreen), a new electronic effect in which elements of the image from one camera replaced areas of the image from a second camera which were keyed to a specific colour (usually blue, green or yellow). Tests for this technique had been held as far back as August 6th, and Letts saw enormous possibilities in its use. All the other scenes for parts two and three were taped on December 22nd, as was the remaining material set in Quinn's cottage from part four, to avoid the need to erect the set again at a later date.

A two week break followed, with recording resuming after the New Year, on January 5th, 1970. The remaining four studio sessions were more traditional in nature, each featuring the taping of a single installment on consecutive Mondays. Episodes six and seven were enacted in Television Centre Studio 8, whereas the earlier recordings had all occurred in TC1; The Silurians was completed on January 26th.

• 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 #222, 15th February 1995, “Archive: Doctor Who and The Silurians” by Andrew Pixley, Marvel Comics UK Ltd.
• Doctor Who Magazine Special Edition #2, 5th September 2002, “Instant Karma” by Andrew Pixley, Panini Publishing Ltd.