In order to summarize a text, one must be able to pick out the key points. The trick to being able to summarize any work of fiction, even a play, is knowing the essential elements of plot development, such as exposition, climax, and resolution. Understanding what these elements are and being able to pick them out will help you be able to zero in on the key details of the story line, enabling you to be able to summarize it.
Any story begins with the exposition. It is the moment when characters and situations, including their problems, are introduced. The characters' problems an important element of exposition because it is their problems that eventually lead to the climax. We meet several characters in the opening scene of the play, which is one moment of the exposition. Among those important characters are Hermia and her father Egeus. We are especially introduced to Hermia's problem, which is that her father is forcing her to marry Demetrius against her will upon threat of punishment by death. We see Egeus petition Duke Theseus for her punishment by death when he, in accordance with the "ancient privilege of Athens," asks for permission to "dispose of her" in the manner he sees fit, which is either through marriage or death (I.i.42-45). While Hermia and her father are central characters with a problem that leads to the climax, the exposition introduces other characters and problems as well.
The climax is the most emotionally intense moment of the story. It is usually the most exciting and dramatic point of the story, and the moment that the crisis begins to resolve. The moment of climax begins when Oberon realizes that Puck has mixed up the lovers, making both men fall in love with Helena, rather than fixing Helena's love problems. At this moment, the four Athenian lovers begin to have a very heated quarrel. Finally, Oberon gives Puck further instructions for remedying the messy situation, ensuring that both couples are paired appropriately.
The resolution, sometimes called the denouement, of a plot occurs when things begin to wind down. It is the moment when all problems are resolved, leading to the end of the story. One element of the resolution of A Midsummer Night's Dream takes place when Egeus and Duke Theseus discover the four lovers in the woods. Against Egeus's wishes, Theseus grants all four lovers permission to marry, as we see in his lines:
Egeus, I will overbear your will;
For in the temple, by and by, with us
These couples shall eternally be knit. (IV.i.180-182)
While there are other elements of the plot, such as the mechanicals' play and Oberon and Titania's conflict, understanding these points of plot structure should help you pick out the rest of the important plot elements, enabling you to summarize the story line.