This episode goes behind the scenes of The Unicorn and the Wasp

The Doctor and Donna travel back to the 1920s and enjoy an unexpected encounter with Agatha Christie. All is well until the party reveals an unwanted guest with a sting in its tail. We find out exactly what goes into creating a classic murder mystery, what it's like behind the scenes with such a big ensemble cast and discover the mysterious events surrounding the real Agatha Christie. Features interviews with David Tennant and Fenella Woolgar.