Meanwhile, the main plot about the mind-control device was something writer Don Houghton intentionally included as an homage to A Clockwork Orange. Behind the scenes, Evil went badly over budget, thanks in no small part to one of Doctor Who's rare usages of a real helicopter in the concluding episode. An unimpressed Barry Letts therefore withdrew director Timothy Combe from his informal "director's rota", and Combe never worked on the programme again. Following the destruction of its colour telerecordings, Evil became the "most missing" serial of the Jon Pertwee era, in that not even a frame of it survived in colour on any broadcast-quality medium. Fortunately, the whole of it remained available in monochrome, due to the fact that some of BBC Enterprises' overseas customers required black-