This is one of the fairy tales that inspired this whole set: in "Night Terrors," the Doctor tells a kid, "When I was your age, about, ooo, a thousand years ago, I loved a good bedtime story. 'The Three Little Sontarans.' 'The Emperor Dalek's New Clothes.' 'Snow White and the Seven Keys to Doomsday,' eh? All the classics." That last one being a reference to not just "Snow White and the Seven Dwarfs," but also the stageplay Doctor Who and the Daleks: Seven Keys to Doomsday. This book is kind of like Seven Keys to Doomsday or maybe more the 1964 serial The Keys of Marinus, in that the activation keys to a deadly weapon have been hidden across a planet. However, even considering this is a 36-page fairy tale, the way Snow White recovers them feels a little too easy to be convincing.

Bizarrely, four years before he wrote this, Justin Richards wrote a completely different story with the same title, for Doctor Who: The Official Annual 2012. More here.