The Doctor and Donna travel to December 1926 and meet the renowned murder mystery writer Agatha Christie (Fenella Woolgar), who is attending a party at Lady Eddison's (Felicity Kendal) country manor. They investigate a jewel robbery perpetrated by the "Unicorn" (Robina Redmond, portrayed by Felicity Jones) and a spree of murders committed by an extraterrestrial wasp (Reverend Golightly, portrayed ...