One of the best markers of success in having a historical figure on Doctor Who is when the episode they appear in perfectly encapsulates who they were as a person. In that regard, The Unicorn and the Wasp is a wonderful exploration of Agatha Christie, both as a writer and a woman. It's also a well-written murder mystery that's a throwback to the type of stories Agatha herself was famous for (in fact, that's a key plot point in the episode). It features probably one of the funniest scenes in Doctor Who history, with the Doctor and Donna playing a frenetic game of charades that is doomed to hilarious failure almost as soon as it begins. Overall, it's just a textbook example of how to do historical episodes -- The Unicorn and the Wasp feels firmly rooted in its era, rather than just an arbitrary time period chosen just to make a historical. We can't say the same for some of the Victorian stories we've been getting of late.