This is an excellent opening to a story. I love mysteries, and this one got me engrossed from the start. The characters are really, really good and the art is lovely. The plot isn't complete as this is clearly the first part of a 2-parter, but what's here is very good and the cliffhanger is strong. I also like how it's tying into the series arc. I think the way in which each story in this run builds upon the others works really well to form a cohesive narrative that feels like a season of television that could have been showran by Steven Moffat. It's great.