Transmission: 31.10.2015/ 07.11.2015.

Writer: Peter Harness.

Director: Daniel Nettheim.

Guest Starring: Jemma Redgrave; Jaye Griffiths; Ingrid Oliver; Rebecca Front; Aidan Cook; and Tom Wilton.

The Zygons are back – and so is Osgood, the Doctor's biggest fan. But she's dead... isn't she?

We're officially in the second half of Series 9; episodes 7 and 8, and the third two-parter of the run (or fourth depending on how you categorise The Girl Who Died and The Woman Who Lived)! This one has an international scale: because the Zygons are amongst us. Everywhere.

Following the events of The Day of the Doctor (2013), there's an uneasy peace on Earth. Humans and Zygons live together – but not in perfect harmony. The Doctor and Clara are called in by UNIT when Osgood, somehow now alive, is kidnapped by a troublesome faction of the shape-shifting aliens.

We're on the brink of a global crisis, and the Doctor has to learn that peace is never easy.

Rebecca Front's Colonel Walsh sets out the problem quite succinctly: "Any living thing in this world, including my family and friends, could turn into a Zygon and kill me any second now. It’s not paranoia when it’s real."

"If it's about anything," Peter Harness told Doctor Who Magazine, it's about the difficulty of maintaining a ceasefire. That's something Steven [Moffat, showrunner] really drew out of it; how in a very realistic way, in a very human way, how difficult it is to stop people fighting each other."

Harness returns to the series, fresh off the TV adaptation, Jonathan Strange & Mr Norrell, and showrunning Wallander; last year, he contributed probably the most controversial episode of Doctor Who Series 8, Kill the Moon. This year's contribution is very different, however: more of a political thriller... with an added injection of Invasion of the Bodysnatchers.

Peter Capaldi is particularly enthusiastic about having the Zygons back. He told Blogtor Who: "I was very lucky, I made a film many years ago with a very little part called Dangerous Liaisons with Glenn Close and John Malkovich which was set in the 18th Century and had the most beautiful costumes. The costume designer was James Acheson, who had created the Zygons. Because he worked on Doctor Who, before he went on to great success and acclaim in the movies, all I wanted to do was talk to him about Zygons! He created those things with limited resources; I think it’s a great testament to his talent."

Director, Daniel Nettheim is new to Doctor Who, although he did work on four episodes of K9 in 2010. Further credits include Line of Duty, Humans, and Whitechapel. He's responsible for giving the show an international feel, with a narrative that takes us to New Mexico, through London, and to the fictitious Turmezistan.

UNIT is back, spearheaded by Kate Stewart who we last saw in the series opener, The Magician's Apprentice – and Osgood is a further returning face. Yes, she died in 2014's Death in Heaven, but when has death ever got in the way of a good story? With shape-changing aliens involved, for anyone thinking they've got the solution to her resurrection down to a tee, Peter Harness says, "It isn't quite that simple..."

The Zygon Invasion airs on BBCOne at 8:15pm on 31^st October 2015, only the second episode of Doctor Who to ever screen on Hallowe'en!