Partners In Crime is a significant episode in many ways, most notably for the reintroduction of Donna. It would not be nearly as fun however, without the witty and nutty Miss Foster. Russell T. Davies tongue in cheek take on the bad nanny theme is in turns both unnerving and hilarious. Sarah Lancashires performance is a bit over the top but it needs to be to compete with so much absurdity. She comes across as a twisted Mary Poppins who is sacrificed by her clients at the end despite the dedication she shows her charges. Miss Foster is not an overtly evil character but more an opportunistic one. Her moral ambiguity is what makes her so interesting. The best villains are drawn in shades of grey. They exhibit complex or even conflicting motivations and are richer for it. An immoral character is easy to hate but an amoral one creates room for doubt and sympathy. The audience is invested in the character because they are trying to figure them out and decide how they feel about them.