Doctor Who: Watch Previews for Next Week's Episode "The Power of Three"

The third episode of Doctor Who's seventh series, "A Town Called Mercy," has just finished airing in the UK. And while American fans of the long-running British sci-fi series still have a few hours to go (as of this writing) before they can see the Doctor's western outing, they can at least be treated to some glimpses of next week's episode, "The Power of Three."

Out of the five episodes slated to air this year, we've probably been told the least about "The Power of Three." All we knew for sure, prior to these trailers, was that it would follow a strange invasion of small black cubes which would mysteriously appear and begin to count down with glowing numbers. Beyond that? Nothing concrete.

The two trailers -- one that aired right after "A Town Called Mercy" and a standalone teaser -- give us a little more insight into what exactly is going down. It's the penultimate episode for regular companions Amy (Karen Gillan) and Rory (Arthur Darvill), who will be departing the series after the fifth episode, "The Angels Take Manhattan." As such, this feels very much like a swan song episode for the duo. The trailers show them examining their lives as they've been impacted by the Doctor, realizing that their involvement with him has essentially led to them leading incompatible double lives, between which they'll have to choose. Of course, that becomes less easy, considering the fact that the Doctor comes to live with them for a year after the mysterious invasion of black cubes.

We're shown the return of U.N.I.T., who invade the Pond's house while Rory is in his underpants, and we're also shown the return of Rory's dad, Brian (Mark Williams), who was really quite memorable in the second episode of series 7, "Dinosaurs on a Spaceship."

As for what the cubes are doing on earth, all we really are given are some shots of a spaceship outside earth, a pretty nasty looking alien guy, and a girl whose face suddenly begins to turn blue. Given that the answer is probably really complex, it's probably not beneficial to us to speculate (though if you'd like to, feel free to share your thoughts in the comments).

You can check out the two trailers below.