this story features the only use in "doctor who" of a camera-linking system known as scene-sync that allowed the use of non-static shots of characters superimposed onto a miniature set. as the cameras on the actors were moved, the cameras on the miniature set moved the equivalent scaled amount automatically. the exact scale motion was achieved by trial and error, involving minute adjustments to the voltage delivered to the slave camera's motors. part four's closing theme was transposed to the lower key of the original theme music.