The last time you posted this essay, it was going to be about perceptions of obese people. If you have changed it to a simple review of an episode of this show, this is a good introduction. It gives a brief outline of the series overall and a general statement about the theme of the specific episode. I will focus my comments on details since the overall content is good.
In the first sentence:
“Shows” should not be plural. Just give the title or say “The TV show 3rd Rock…”
In US English, we would call this “a half hour” show, not “a half an hour” show. Unless this is different in Australia, that is a change you should make.
“Channel 20” is a proper noun and should be capitalized.
In the second sentence:
“Sitcom” should be “sitcoms.”
“Genre” should be deleted unless you say “… is one of the most successful shows in the fantasy sitcom genre…”
In the third sentence:
“Forecasts” is not the right word. “Depicts” or “portrays” would be better.
The rest of the paragraph is quite well written, which brings me to a final concern. From what I have seen of your writing over the three essays you have posted here, this does not sound like you. Please be very sure that you have not copied any sentences from other sources. If you have, please be sure to cite your sources.