A preview of next week's episode of Doctor Who plays out one of the Doctor's greatest fears: planet Earth invaded by the Zygons.

In a new video released by the BBC, the Zygons aren't in the process of invading Earth, though, because they've already infiltrated the planet, by disguising themselves as humans.

The Zygons, who first appeared in the 1975 episode "Terror of the Zygons," have shape-shifting abilities, which lets them take on the form of other creatures. However, there's a catch: a Zygon can only remain in that form while the original subject remains alive.

The preview also features the return of Osgood (Ingrid Oliver), who warns the Doctor (Peter Capaldi) of the aliens and their plans for taking over Earth.

There's just one problem: Osgood died in "Death in Heaven" at the hands of the Doctor's number one nemesis, Missy (Michelle Rodriguez). So how did Osgood return? Is Osgood, in fact, a Zygon?

Here's the official description of "The Zygon Invasion":

"The Zygons have been living in secret on Earth, unknown and unseen - until now. When Osgood is kidnapped by a rogue gang of the shapeshifting aliens, the Doctor, Clara and UNIT will have to scatter across the world in a bid to set her free. But will they reach her in time, and can they stop an uprising before it's too late?"

This episode also sees the return of Clara (Jenna-Louise Coleman), noticeably absent from the past episode, "The Woman Who Lived." However, she disappears again for the second part of "The Zygon Invasion." Here's the official description for that episode:

"The future of planet Earth is sealed in a box in Unit's back archive, and only the Doctor knows what's inside. With Unit under Zygon control, and Clara lost, the Doctor and Osgood find themselves fugitives in a London where no one can be trusted - but the wily old Time Lord knows there is one last hope for peace. Because that box in the black archive isn't any old box. It's an Osgood Box!"

However, there still is the question about how Osgood returns to the series, unless of course, the answer is simply a one of wibbly-wobbly timey wimey (as in time travel). Or perhaps the mystery lies with the previously-mentioned "Osgood box" - what exactly is it?

Doctor Who airs on BBC America on Saturdays.

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