"Blue Forgotten Planet by Nicholas Briggs, is India Fisher's swansong as Charley Pollard. Once she hooked up with the Sixth Doctor after her adventures with the Eighth, it was clear that some pretty dramatic resolution was needed, and the stakes were raised a couple of stories ago by the introduction of a fake Charley identical to the real one. Briggs brings back the virus-hunting Viyrans, who he introduced for a series on one-episode stories a while back, for a resolution which barely works in plot terms but pays off very well emotionally and dramatically. Fisher is brilliant as the two versions of Charley - we are rarely in much doubt as to which she is acting. The setting of a devastated post-apocalyptic Earth is superbly portrayed, and the fact that the few surviving sane humans are making nature documentaries (with lovely background music) about their blue, forgotten planet, seems tragic rather than ludicrous. I didn't find it quite as good as Paper Cuts, but it is pretty good. (Unfortunately one of the guest cast, J.J. Field, takes most of the first episode to wake up to the fact that he is supposed to be acting.) See also Why Charlotte Pollard Pwns Rose Tyler."
I'm really conflicted about this one because the "Charley" arc didn't sit well with me... but regardless of that, good adventure with very interesting villains, and a satisfying, if heartbreaking, conclusion to Charley's arc.