Producer Jonathan S Powell said: "Zygon Century is back - and the invasion's gone international! After the success of Infiltration, Jonathan Barnes and I were keen to push ourselves further and show our band of Zygons on the offensive. In these three exciting new episodes, we'll take you across the twentieth century, from Russia to Dublin to Death Valley, and reveal more of the Black Cadre's simmering masterplan.

"I'm immensely proud of Zygon Century - it's Doctor Who but it's also proper character drama, and goes places the main series can't. There are many more stories still to be told, and we hope you enjoy these next three.

"This time around we're welcoming not one but two iconic Doctor Who characters into the Zygon Century fold. But what does the Master want with the Black Cadre? And what on earth is the Third Doctor doing in Soviet Russia? Schemes are unravelling, destines are converging, and all roads lead to world's end. But is humanity ready?"

Script editor and writer Jonathan Barnes added: "I was very keen to come back to our three main characters for the second box set. So we have Herbert Scott, the occult investigator, Vorvoren, the most sympathetic Zygon in the entire series, and Agent Caldwell.

"Not all of these people will survive to the end of the series, and there's going to be many unexpected developments in their stories going forwards. One of the great advantages of writing a Doctor Who spin-off is that we don't have the Doctor for great swathes of these stories, so we have protagonists whose destiny is unclear. That's exciting to work with."

Actor Jonathan Rigby describes his character, Herbert Scott: "In the previous volume, he was in his prime, as a rather equivocal, slightly dodgy occult investigator. Now he's been banged up in the asylum and we meet him again 21 years later. Because of his experience with the Zygons, he is unchanged physically, but of course he's had a very distressing 21 years. The disorientation of suddenly being back in the world is a fascinating part of the story."

Charlie Russell, who returns as the Zygon Vorvoren, said: "I'm so happy to see her development. She's seen the worst of the human race, the really sharp end of that stick, and it's hardened her and made her less trusting, but also still vulnerable. And it was so exciting to meet the Master. He brings out of her the fact that she is so angry and full of desire for vengeance."

On the Third Doctor’s Zygon Century debut, Tim Treloar said: "The Zygons have infiltrated the Soviet Union's nuclear power capacity so the Doctor's there to give a hand. There's very much a James Bond vibe going on. I love the fact it's set in the 70s at the height of the Cold War. Caldwell and the Doctor are a bit of a double act, on disagreeable terms!"

Duncan Wisbey, Leonie Schliesing, Jo Herbert, Tim Treloar, Joshua Manning, Deirdre Mullins, Branko Tomovic

James Swanton, Toby Longworth, Abigail Thaw, Jonathan Rigby, Nina Georgieff, James Gaddas