This one is better than I remembered!

Silence in the Library is stuffed with enough sci-fi concepts to nourish several other stories: a planet sized library; invisible monsters in the shadows; data ghosts; Donna's fictional children; Donna tragically not crossing paths with her real husband. But the two best concepts of all are The Doctor having a wife from the future who he hasn't crossed paths with, but knows him better than anyone in the world; and a there being child who is a computer in her own world watching the real world unfold on her TV.

Catherine Tate plays her part incredibly well here, from the two hander with David up top, to caring for the bullied member of the crew to her disorientating journey through the dream world. Alex Kingston also smashes her introduction as the infamous River Song - the concept of her story is so deep and broad that it feels impossible at this point that they would even manage to bring her back. Props should also go to Steve Pemberton who (as also seen in Happy Valley) can twist from malicious to sympathetic in an instance, giving his character an enjoyably multi-layered performance.

This is Moffat at the height of his powers, not serving up needlessly complicated plot in order to wow the audience, but instead carefully introducing thread after thread of different concepts, gently crafting them into a satisfying whole that totally adds up.