A look at cult sci-fi series. It took producer Richard Bates 15 years to get John Christopher's trilogy The Tripods on to our screens. Then, when he did, he only got to make two-thirds of the story. But despite it never being finished or repeated, its popularity has sustained. Including interviews with cast and crew, the programme reveals how the casting of the three leads turned into something close to X Factor, and how a piece of BBC weather department technology changed the approach to SFX.