This is a real judgement/ interpretation call with any number of possibilities. I think los tantos can refer to either where all the characters involved come together or where all the events take place. Or both in making up the totality of the film/story. I gave you the broadest, most generic answer that covers most possibilities.
The kicker is se invierten. There is the phrase "se invierten los papeles", which could mean the "roles are reversed", "the tables are turned" or less literally "things get turned upside down." Which one could apply here depends on the specifics of your text and the story of the film.
Hope this helps for the general idea and you can tailor your specifics.

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Note added at 20 mins (2011-03-28 14:39:30 GMT)
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Or you could say "all hell breaks loose" to be more colorful if that's what fits the story. Any number of ways you can go with phrasing it.

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Note added at 27 mins (2011-03-28 14:47:00 GMT)
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"and where all the action takes place" could be another good general answer.