When summarizing a piece of literature, such as a story or a play, it's important to first know how to pick out the important details. When it comes to stories, we can actually use what we know of typical plot structure to help us notice what is important. A typical plot structure includes five elements: the exposition in which characters are introduced; the rising action, which involves all actions that develop the conflict and lead up to the climax; the climax, which is the most intense moment of the conflict; the falling action, which involves all actions that lead up to the resolution; and finally the resolution, which is the ending to the story. If we keep these elements in mind and know how to pick them out, then we know how to pick out what is most important and can quickly summarize the story.
The exposition takes place in mostly the first act with a little still developed in the second act. In these two acts, we meet all of the principle characters, including but not limited to Duke Orsino, Viola, Olivia, and Sebastian. In addition, we are introduced to the play's conflicts. We learn of the first conflict in the very first scene, which is that Orsino is obsessively pining for Olivia who has rejected him, not wanting any company since her brother's death. We are introduced to the second conflict in the second scene and learn more about it in the fifth, which is that Viola feels the need to disguise herself as a manservant, allowing the opportunity for Olivia to fall in love with Viola as the manservant Cesario while Viola has fallen in love with Orsino. Any scenes developing these conflicts also serve as the rising action, such as the moment when Malvolio hunts down Cesario to return a ring Olivia says the servant left with her, allowing for Viola to realize that Olivia has fallen in love with her as Cesario.
The climax starts in Act 4, Scene 1, the moment that Olivia mistakes Sebastian for Cesario. This serves as the climactic moment because, from that point on, Viola's true identity will inevitably be revealed. The climax continues in the fifth act when Sebastian and Viola see each other, exposing Viola's real identity and revealing that Olivia has married Sebastian rather than Cesario. This final scene also serves to develop the play's resolution, which is Olivia's marriage to Sebastian as well as Orsino's proposal to Viola. Now that Viola has been exposed as truly being a noblewoman, Orsino, having fallen in love with her as Cesario, decides to ask Viola to be his wife, as we see in his lines:
Your master quits you; and, for your service done him,
So much against the mettle of your sex,
And since you call'd me master for so long,
Here is my hand: you shall from this time be
Your master's mistress. (V.i.327-34)
Hence, we see we can easily summarize the story by first taking note of the important events through examining the plot structure.