A three-act structure is one particular way in which a story can be told and is often employed when writing a screenplay for use in film. This basic format breaks up the story into three pieces, each called an "act," and though some writers find it flawed, this structure works well for many screenplays. The first act introduces the characters and the problem faced by them, presenting a moment at which point the main character sets on his or her path. In act two, this problem intensifies and nearly seems overwhelming, until a climax is reached and the problem is solved in act three.
The first act of the three-act structure is typically described as being the first fourth of a story; a 120-minute film would have a first act of about 30 minutes in length. In this section, the main and supporting characters are introduced and the basic problem of the story is presented. For example, if a hero needs to free a land from a cruel tyrant, these characters are shown and the audience sees how evil the villain is. In a three-act structure, the first act ends with a plot point that usually consists of the hero taking on the problem, so in this example the hero might set out to find and defeat the evil king.
Following the plot point, the second act of a story in a three-act structure begins, as the hero sets off to solve the problem. This act is usually half of the story, so it might be the middle 60 minutes of a 120-minute film. During the second act, numerous obstacles arise that prevent the hero from solving the problem, such as groups of creatures that must be defeated before reaching the evil king's castle.
At one point in the second act, all hope seems lost and the characters believe that success cannot be accomplished. This is often called the midpoint and falls about halfway through the story in a three-act structure. They come through this, however, and continue moving with renewed purpose toward the plot point that ends the second act, such as them reaching the tyrant's castle.
The final act within the three-act structure is the last fourth of the story, or about the last 30 minutes of a 120-minute film. In this third act, the problem reaches its climax at the moment of greatest dramatic tension, such as the hero facing the evil king in personal combat and defeating him. Once this occurs, the problem is essentially solved and the remaining story is a resolution, such as the hero returning home to rejoin now liberated loved ones. The rest of the three-act structure allows the audience to return to normalcy and enjoy the victory of the hero over the problem.