Screenwriter Robert Towne says a film really boils down to just five key moments.
Everything else? Every other page? Every other scene and line of dialogue? They exist to give those moments their impact and resonance.
Inciting incident. This is the first premonition of impending trouble, dilemma, or circumstance that will create the main tension of the story. This is when the story really starts. Usually at the end of the first sequence but sometimes in the first few minutes. Pressure starts.
Lock-in. The protagonist is locked into the predicament that is central to the story and there’s no turning back. This occurs at the end of the first act and propels the protagonist into a new direction in order to accomplish their new objective throughout the second act. Pressure stokes.
First culmination. Generally occurs around the midpoint of the second act and is a pivotal (but not the most pivotal) moment. It’s the second-highest or second-lowest point in the second act, the second-highest hurdle the protagonist faces. Pressure builds.
Main culmination. Occurs at the end of the second act and brings the main tension to a close while simultaneously helping to create a new tension for the third act. Everything ratchets up to a new level of stress. Pressure mounts.
Third act twist. This is an unexpected but all too believable turn of events in the third act. Without a twist, the third act can seem too linear and predictable. Typically it’s the last test of the protagonist. Pressure releases.
The story ends after the point of catharsis, where the protagonist and the audience leave cleansed and purified.
Where they leave ready for a new beginning.
Ready for a new story.