Steven Moffat says bringing back the Zygons has been an ambition since he took over Doctor Who – and that the classic monsters are so well designed he hardly had to change a thing for their return in the 50th Anniversary Special.
"Every year since I took over [Doctor Who] I've been trying to get the Zygons in," says Steven Moffat, "and then I thought 'Well, it's the 50th...'
"The Zygons are beautifully designed monsters, they are so wonderful... We barely changed the design at all because it was so good."
The classic Who foes have appeared just once before, in 1975 adventure Terror of the Zygons, yet remain a firm fan favourite.
Moffat said there was a certain symmetry in bringing them back alongside the Doctor's most famous foes the Daleks, who will also star in special episode The Day of the Doctor.
"Obviously the Daleks had to be there, the Daleks had to come to the party," Moffat told the audience at a Radio Times-sponsored event, "because they’re really scary and are part of the iconography of Doctor Who – possibly the central part – so we had to have them.
"And in a way it was fun to bring back Doctor Who's biggest monster success ever, the Daleks, and then go to maybe the smallest monster success [the Zygons] – they were only in it once but everyone remembers them – great monsters, great outfits... and they have these nice whispery voices..."
Terror of the Zygons saw the aliens invade earth with the help of a rather dodgy looking cyborg monster that lived in Loch Ness Monster, and which Moffat jokingly said was "incredibly convincing" and "still seen by special effects experts as their finest achievement."
The story saw the Doctor fighting alongside Brigadier Lethbridge-Stewart and Unit (Earth's Unified Intelligence Taskforce). The Brigadier's daughter Kate Stewart (Jemma Redgrave), head of scientific research at Unit, is known to feature in the 50th Anniversary Special and fans will be hoping that means a full-blown rematch between Unit and the Zygons – and perhaps the return of the Loch Ness Monster...
The Doctor Who 50th Anniversary Special, The Day of the Doctor, will air on BBC1 and around the globe on Saturday 23 November.