In every season of new Doctor Who there is a meet-the-famous-author episode, and Donna's is a light little romp through the works of Agatha Christie, with the perfect Fenella Woolgar playing the dame herself at the beginning of her career... and at the beginning of her real-life famous disappearance.  It's the classic country house murder mystery: a series of people have been invited to a weekend party in the country by Lady Eddison, played by Felicity Kendal. (If you're an American staring at her thinking "I know her from somewhere," she was the wife from Good Neighbors, a 70s Britcom that has been the staple of many a PBS station.)  Among the guests are Agatha Christie, a shy country vicar, a daring society daughter, and Lady Eddison's family.  There's also a professor, but he is bludgeoned to death early on.

In the library.

With a lead pipe.

In a plot that is as much Clue/Cluedo as Christie, the Doctor and Donna are off and running, trying to figure out the secrets of their fellow guests, at least two of whom aren't who they pretend to be.  One is the jewel thief "The Unicorn."  The other, this being Doctor Who, is a giant shape-changing wasp.

The plot is littered with Christie references to books both published and at that point unpublished.  It also smacks a bit of the casual racism of the time (the Eddison's Indian housekeeper gets a very stereotypical death, which has led to complaints from some fans.)  But all in all, it's silly light fun.  Old school fans will be strongly reminded of the Fifth Doctor episode The Black Orchid, which was always a favorite of mine.