The penultimate episode of Series 8 is vastly entertaining. Although fans guessed Missy = Mistress = Master weeks earlier, the final reveal of her identity still works (Capaldi's reaction sells it). Throughout this episode, events gather pace in a natural manner. Danny's surprise fatal accident, Clara threatening to melt the TARDIS keys, the Doctor and Clara meeting Missy, the watery dead turning out to be Cybermen and Missy blurting out her secret - all of these parts are honestly interesting and engaging. And when a Cyberman invasion story shows the silver giants marching out of St Paul's Cathedral, fans can only grin wildly. It's always easier to build up the threat in a story than to resolve it, which is why Dark Water is better than Death in Heaven. However, Dark Water deserves to be appreciated on its own merits. Apart from everything else, it provided viewers with a genuinely excellent cliffhanger, which is something that Doctor Who had lacked for quite a while.