That was just the most funtastic Doctor Who Episode so far this season. "The Unicorn and the Wasp" Set in 1926 and included Agatha Christie. Set the night before she mysteriously disappeared lent itself to a great opportunity for a Doctor Who story. And why not make it a Murder Mystery. With lots of characters (straight from the Cluedo Board game pretty much) and some fun play on words with Christie novel titles, right from the opening scene "Why didn;t they ask.. Heavens" to Sparkling Cyanide, Dead Man's Folly and many many more!
Great casting, great scripting, great Charades (A Harvey Wallbanger?) and some tongue in cheek word play.... Ginger beer? all adds to this fantastic episode.

But didn't it remind you of Black Orchid?