Travelling with the Doctor, Clara’s seen loads of aliens, large, small, friendly, hostile, fluffy, scaly, and everything in between. Some were even so like humans it was hard to tell the difference just by looking at them.
She’d crossed paths with some of the Doctor’s most implacable enemies, Daleks, Cybermen, and Sontarans among them, yet however alien they were she’d been able to understand their motives, at least a little.
But this time they’re up against something that defies comprehension. Two-dimensional beings have found their way into the three-dimensional world, and even the Doctor has no idea what they want.