6. Torchwood (Series Two)
Another series arc that was barely an arc so much as an occasional throwaway line of dialogue; Torchwood seemed like little more than producer Russell T Davies' cynical way of promoting the imminent spinoff series of the same name although why anyone would want to make children aware of a show that regularly employs gratuitous sex and usage of F and C bombs is beyond me. The alien-fighting institute is first mentioned by name in David Tennant's debut story The Christmas Invasion, then its inception is seen at the end of Victorian horror story Tooth and Claw. And then... that's it. Torchwood gets a fleeting mention here and there, but it has no impact on the stories which mention it and isn't really thought about by anybody until the finale rolls around and we finally see what Queen Victoria's brain child has become. This style of threading episodes together with a seemingly throwaway line actually worked a lot better with the preceding series' 'Bad Wolf' meme, because that time around it was abstract, subtle and a little bit spooky. But more on that later. The word 'Torchwood' being shoehorned in amidst discussions about Cybermen and blackholes doesn't quite constitute an arc, although at least it didn't result in an under-developed companion.