I believe it depends on where in the German language area you would say it. I understand that the past tense is still alive in its northern parts, though I can't say whether a speaker would use it in the example you gave. In the south it would certainly sound stilted.
That said, the most natural way to talk about a film is to just say:
Der Film handelt von ...
since the film wasn't about that only that one time you watched it anyways.
In Austria, there are only two cases of past tense use that I know of (in spoken language), which are the past tenses of "sein" and "wollen": "Ich war ..." and "Ich wollte ..." sound perfectly natural. With all other cases, you would use the Perfekt, "Ich habe gedacht..." etc. This will vary depending on your location htough.