This is a brilliant, yet rarely seen classic. I have managed to retain for myself a grainy VHS copy taped off UK Gold about 20 years ago. First thing to say, it is directed by the late, great Barry Letts. The second thing is that there are one or notable exits in the story - the amazing John Levine and the tragic Ian Marter both make their last appearances as Benton and Harry Sullivan respectively. Written by Terry Nation it has a feel of an Avengers Episode with dopplegangers, people behaving oddly and mysterious research establishments. It has one or two plot holes in it but they are more than compensated for by the brilliant Milton Johns who is just ever so slightly miscast as Guy Crayford, experimental deep space rocket test pilot
