Unoriginal to say the least. Take Little Red Riding Hood, change Red to Rose, done. It is, I admit, amusing to throw in the Bad Wolf reference, and a Zygon looking like Grandmother is much more convincing that a kid not being able to tell the difference between her gran and an actual wolf. But still, what was the Zygon's reason for impersonating Granny? How do we explain Rose meeting the Doctor before Rose meets the Doctor? Seems lacking.