The premise of this one is relatively simple, but tends to become complicated when it is explained. The Master gains access to Nestene intelligence which allows anything plastic to become dangerous. It's really as simple as that. You can get as sci fi about as you like.
But of course that is not how I would approach it - the premise of dangerous plastics allows endless japes, like murderous toys, deadly flowers and chairs which eat people. Oh, and plastic police officers. You can approach this one as horrifying if you want - in fact it was given in Parliament as an example of how children's television had become scary - but watched as an adult, it is a jolly romp.
This Who calls in a feature of the TV of the sixties which I bang on about here - the ambivalence about the bright new scientific future which was otherwise all the rage at the time. Two points about this are made in the special features of the disc - that again this was horrifying because it made something dangerous which is found in every home, and that there was a fear this storyline would clash with Doomwatch's line about plastic deteriorating. It is commented that this fear was ungrounded because Doomwatch was completely serious, so perhaps I am not too far off in my approach to this show.
Blue screen filming is used extensively here, both to allow the effects but also for many scenes to give a backdrop. Of course it was the technology of the time, and can look very old fashioned. Otherwise the adventure is paced perfectly and four was the perfect number of episodes.
Can anyone reading this not have seen this? But if you haven't, do run away and watch it.