Episode S03E12 "The Sound of Drums" and S03E13 "Last of the Time Lords"

Alexandra has a very minor role in these two episodes. Her role is as the wife of a politician who turns out to be the evil time lord "The Master". Its unclear wether she knew of her husbands evil plans. At the start of these episode she is the loving wife of a masterful politician but at end she appears to have been beaten (physically) and acts dazed and confused with a rather vacant expression. Its never explained in the show. Either way she is very attractive.

Lucy Saxon (Alexandra Moen Wiki IMDB)