Let's make a habit of this, as it's new Who TV and thus handy for turning over and looking at.
The Empty Planet
The characters find that the world has been deserted. Why? Why have they been left behind? And if they're alone... why did that lift just open?
(The latter creepy moment didn't make it into the story, having been part of an earlier version that was almost one of the 2009 Doctor Who specials, as discussed in Russell T Davies's great big behind-the-scenes book The Writer's Tale.)
It's a great way of cutting some characters off from others and from their support network, looking at isolation, loneliness and the fear it can bring and connections it can strengthen, giving them a chance to shine while others are absent... and have big robots stomping around.