Diverting although unsatisfactory. The high point is the way in which the story conjures a futuristic yet timeless Venice. The sense of its decline, both physically and metaphorically, is well sketched and along with certain aquatic creatures evokes both Poe and Lovecraft, all with added class war which I admit I'm a sucker for. Sadly these elements don't all pull together for me, and the sci-fi conceit seemingly necessary to turn this into a Doctor Who story feels undercooked and tacked on.