This is one of the first TV series that hooked me as a child. It scared the life out of me, but I loved it and I remembering starting a playground game called 'The Big Pull' which was based on aspects of the show (loosely, I'm sure!) but was pretty popular with my friends while the show was on.
I'd love to see it now, but guess it would look pretty tame and even a tad embarrassing. I was fascinated to see that the show was produced by Terence Dudley who later had such a big hand in one of my other favourites shows - the excellent 'Survivors'.
Another review here remembers the name 'Jotus' and 'Nebula' - something I have not forgotten either. The reason is that one of the dead 'humans' inhabited by an alien, recounts this information with his mouth and eyes wide open and the voice echoing from somewhere inside him. I was terrified when I saw it and have never forgotten the scene although I'm sure it would look pretty ordinary now. I also dimly being aware of the need not to be in pairs with a final episode scene featuring a terrified call from a mother with just her baby for company saying she didn't want to 'become a baby'. I can still picture the final shot in my head now - the two 'heroes' of the show melded together as one alien.
Few ever remember this show (although great to see those who do remember it were as equally terrified as I was!) even though it was on the early evening slot on Saturday evenings, (pre Dr Who ) so it's great to see it mentioned here.