he will knock four times, and we'll refuse to bloody well answer

How much do I loathe Russell T. Davies? Well, actually, I don't, he's always come across as a rather sweet and inoffensive man in interviews, and we of the fangirly persuasion owe him oodles for his resuscitation of the Doctor Who franchise. But, ye gods and little poodles, he does the most horrible things to narrative. I finally dug up the gumption to watch the Tenth Doctor's exit last night, and I'm still picking bits of it out of my teeth. I adore the Tenth Doctor. He's a doll. He's also a damned fine actor, and deserves infinitely better than the illogical pap served up to him in the name of plot. ( Further fulminations cut in the interests of spoilerage. )

Anyway. Thanks, Russell T. Davies. On balance we were probably lucky to have you and I hope you go on to even better things, but I do not grok your personal narrative beliefs, nor wish them well. Above all, stay away from time travel. You don't get it.