The Three Act Story Structure is one of the most basic story structures and one of the most useful, in my opinion. It gives the author plenty of room for inspiration, innovation, and invention while still keeping them on track to an exciting and satisfying conclusion. Some may need a more detailed plan (the 27 chapter structure extrudes this structure into granular detail), but for those who seek only a little order to help them tell their tale, this structure does the job.
Act 1: Setup
Exposition
The main character is introduced in their natural habitat. The length of this section is highly genre dependent. A thriller may not spend more than half a sentence here before the inciting incident while a mystery will often spend a whole first chapter here with the inciting incident (likely a dead body) coming at the end.
The Inciting Incident
This is the thing that starts the hero down the path your tale will take them. Meeting an enthralling (or annoying) new someone in a romance, a thriller protagonist’s spouse/child getting kidnapped, the first body dropping in a horror book.
Plot Point One
Where the inciting incident starts the hero down the path to a new world (sometimes metaphorical, sometimes literal), they don’t understand what is happening until this section that ends Act 1. It is the reveal of the conflict/problem that the hero will have to resolve. It rarely, if ever, means they have a full understanding of the conflict, but at least the current face of it is revealed.
Act 2: Confrontation
Rising Action
The protagonist tries to solve the problem present in Plot Point One. They gather allies and make enemies. They experience minor successes/failures.
Midpoint: Understanding the New World
The protagonist comes to a false understanding of the new world and feels they are on the verge of victory.
Plot Point Two
Defeat and despair. At this point, the hero wishes to return to their old world but is blocked, either by evil forces or more likely good forces who need them to remain and fight on. Even better, they are offered a return home but refuse, vowing to do what they came here to do. All metaphorical or literal dependent on genre.
Act 3: Resolution
Pre-Climax
The protagonist achieves a true understanding of the new world and uses it to rebuild their fellowship and vie for ultimate victory.
Climax
Boss fight. Or in things like romance, the lovers break through the final barrier blocking their becoming a couple. The killer is revealed in a parlor room monologue in a classic mystery. The bomb is defused. Hans Gruber falls from Nakatomi Tower.
Denouement
The final bits to send the reader on their way satisfied. The hero kisses their paramour. The cowboy rides off into the sunset. The horror killer is dead (or is he?).