Although Philip Hinchcliffe's first credit as producer was The Ark in Space, all the stories of season twelve had been commissioned by Barry Letts. "Terror of the Zygons" was therefore the first story that Hinchcliffe had full creative control over, and what a terrific story to kick off his first full season with!
Shape changing alien refugees under Loch Ness, A cybernetic oil rig smashing leviathan, and the Brig and UNIT! It doesn't really get much better, does it? And all filmed against the beautiful scenery of.........Sussex. Ah well, the BBC could scarcely afford an expensive trip to Scotland, as they were still paying off the expensive looking tank visual effect from "Robot". No expense was spared on the impressive looking space-craft interior, however; all those organic knobbly bits look wonderfully alien in the subdued green and orange lighting. Regrettably, the same can't be said of the Skarasen sock-puppet, although to be fair it's difficult to see how the effect could have been better achieved in 1975 and on Doctor Who's budget, and in any case it's the productions only real blemish.
Robert Banks-Stewart’s engaging script was a very early indication of the style and format that would shape the stories of the Hinchcliffe/Holmes era. It’s a really well paced story containing action, intrigue, horror and suspense, beautifully directed by Douglas Camfield and boasting a lovely musical score courtesy of the acclaimed Ivor Novello award winning composer Geoffrey Burgon.
But just as "Terror of the Zygons" was the beginning of an era, so it also marked the end of one. As well as being the final UNIT story until 1989, after eighteen regular appearances between 1968 and 1975, season thirteen would be the final appearance of Brigadier Lethbridge-Stewart until 1983's "Mawdryn Undead". The show would also be saying farewell to Ian Marter and John Levene, although both would appear briefly in "The Android Invasion". Terribly sad to see so many well loved characters leaving the series, but nothing lasts forever, and arguably the best was yet to come.