In Story, McKee suggests that an advantage of three-act structure is that it allows a film to explore the many facets of a topic. The simplest aspect is the contradictory, “the direct opposite of the positive.” (319) There is also the contrary, which is somewhere in between these two extremes. This is often more nuanced, and more difficult to defeat. If the contradictory of love is hate, the contrary of love is indifference.
For McKee, the final aspect is “the negation of the negation,” to steal a marxist term. It is often a twisted parody of the positive, or something that is to the contrary what the contrary was to the positive. If the contrary of Heaven is Hell, the negation of the negation is something that makes Hell look like Heaven. The Simpsons had an example in a gag in which the Grim Reaper had his own Grim Reaper.
McKee strongly believes that the story has to reach this moment, and that it takes a minimum of three acts to do so. (332) Usually the theme transitions from positive to contradictory to contrary to the final stage. In some cases, the characters transition from the opposite direction, going from the negation of the negation to the positive version of a theme. This often happens in comedies as deluded protagonists learn to be truly happy.
Thus each act is necessary to explore the various facets of the theme. If law is the theme, the contrary could be crime, and the contradiction would be the grey areas. The negation of the negation could be a tyranny, a consistent system in which the law is the problem with society. If the theme is truth, the contradictory would be a lie, the contrary would be white lies and the negation of the negation would be self-deception.
It makes sense to save the negation of the negation for the third act as it’s when the protagonist is forced to make the most difficult decisions. In TThe Dark Knight, it’s when Batman has to fight Harvey Dent. In Casablanca, it’s when Rick has to choose between the girl and the war. In The Empire Strikes Back, it’s when Luke Skywalker discovers the truth about Darth Vader.
Scriptlab breaks a plot down to five moments, including a third act twist. I’d quibble, as sometimes it seems that twist marks the end of the second act. But it fits McKee’s ideas on the thematic journey, as the twist is often the development that forces the protagonist to question what was once taken for granted.
There was an interesting thematic journey in The Avengers. In the first act, it seemed like a simple Good VS Evil story, although things are muddied in the second act, as the heroes learn that SHIELD, a government agency has been lying to them, and trying to construct dangerous superweapons. In the third act, agents of the government order Manhattan nuked, and the film ends with Nick Fury is comfortable with the team’s autonomy.