Aired 30 August – 20 September 1975

If Tom Baker’s debut story, ‘Robot,’ set out to provide an air of familiarity as the Fourth Doctor tackled a strictly Third Doctor story, ‘Terror of the Zygons’ provides a definite point of departure for the two eras as it plays upon past conventions while injecting the wholly different feel of the Fourth Doctor era that has already become so clear and strong after some experimentation during Baker’s first year.

Even with the Third Doctor continually returning to Earth after his exile ended, the Fourth Doctor made it abundantly clear from the outset that he had no interest in doing so, and the tightly-connected stories of Baker’s first year support this new incarnation’s desire to once more travel through space and time. Thus, his frustration at the Brigadier summoning him back to Earth to help with another alien invasion is understandable, and it’s quite interesting to note how the underlying air of friendliness that simmered beneath the Third Doctor’s condescending remarks towards the Brigadier has almost completely vanished.

As a new foe, the Zygons make an instant impression, buoyed by superb costuming that captures the organic alien menace perfectly. The last survivors of a dying world, the Zygons here have taken refuge on Earth and are able to become duplicates of nearby human beings to better disguise themselves among the general populace. Additionally, the fact that the Zygons brought the Loch Ness Monster to Earth as an embryo to provide the lactic fluid they so desperately need to survive is intricately interwoven into the Scottish backdrop wonderfully. However, the importance of this creature to them does inherently bring into focus the fact that they use it as a key piece in their invasion strategy to take over the Earth, undoubtedly and unnecessarily exposing it to extreme dangers. With the ability to duplicate, there are assuredly many safer ways for this dying race to achieve their goals, and both Harry and the Doctor seem rather unimpressed with the Zygons’ scheming in general, the latter noting that it’s impossible to rule a world in hiding as they want to, especially once the Loch Ness Monster is deployed to murder the Prime Minister.

Indeed, even with the exaggerated Scottish setting and denizens, ‘Terror of the Zygons’ is a superbly-crafted invasion story that isn’t afraid to have a little bit of fun with the programme’s former earnestness in familiar situations. Though the Zygons’ boasting may not hold up to much scrutiny and questioning, there’s no doubting how unique and influential their first appearance is, easily earning them a spot among the best Doctor Who foes. ‘Terror of the Zygons’ feels like a farewell of sorts to the type of story so prominently focused in the Jon Pertwee era, but it does so affectionately while also including the perfect mesh of horror and humour that has already become such a staple of the Tom Baker era. As the Doctor never bothered to formally terminate his employment with UNIT as scientific advisor, ‘Terror of the Zygons’ also proves to be the perfect blueprint for any future stories should the Doctor ever choose to take up that mantle once more.