Sarah Jane: Prisoner of the Judoon..
We first met the Judoon in the Doctor Who episode "Smith and Jones", and then they turned up again in "The Stolen Earth". What a joy to see them again in The Sarah Jane Adventures - where the humourless Judoon Captain Tybo plays straight man (er, alien) to Rani and Luke's jokes.
But the joy of this story was Elisabeth Sladen's acting. In the course of the story, Sarah Jane is possessed by an evil alien, and Elisabeth Sladen does a wonderful job of switching back and forth between the two characters - between menace and heroism.
All this, and nanotechnology too.
I see that Phil Ford, the writer, is also the writer for the upcoming Doctor Who episode "The Waters of Mars". That bodes well.