In this final programme of Series 5, Stephen Fry explores the eXtraordinary story of the 24th letter of the English alphabet, starting in ancient Greece and ending with exploitation movies carrying X ratings. On the way we hear about how x was used as a signature, its place in algebra, marketing and medicine, and, with the help of Gyles Brandreth, why X is a mixed blessing in the English version of Scrabble. We hear the story of physicist Willhelm Roentgen who discovered a luminescent ray that appeared to have immoral properties, and Mark Kermode explains to Stephen why marking movies with an X made them seen more attractive to the very audience they were designed to exclude. And we end with some affectionate xxxx kisses on the bottom with a mathematical explanation as to how and why you gottem and what they mean, courtesy of Rene Descartes and Fats Waller.