It's midway through series three, which saw the return of the Daleks, Jack Harkness and the heralding of a new assistant.
In Human Nature/The Family Of Blood, David Tennants Doctor is hiding out in 1913, in human form, having zapped his brain into believing that hes a mild-mannered schoolteacher.
Long-suffering Martha (Freema Agyeman) must watch him fall for Jessica Hynes (formerly Stevenson) and try to save the day on her own. Its a decent two-parter, but eclipsed by the third episode, Blink, which owes a debt to Back To The Future as a girl goes back in time and finds a way to communicate with her friend in the present. Although, given the Easter egg plotline, we were a bit let down not to find any on the disc.