In the palace in ancient Athens, Duke Theseus and his fiancé Hippolyta are planning their wedding festivities when Egeus, an Athenian nobleman, arrives. Egeus has with him his daughter, Hermia, and two men, Lysander and Demetrius. Egeus wants Hermia to marry Demetrius, but Hermia loves Lysander. Egeus asks Theseus to uphold Athenian law, which forces a woman to marry the man her father chooses or be executed. Lysander points out that Demetrius is an inconstant lover, who had until recently loved Hermia's childhood friend Helena before falling for Hermia, but Theseus says he must uphold the law, and exits. Lysander and Hermia decide to elope by escaping into the forest outside Athens. They tell only Helena of their plans, but she tells Demetrius in order to try to regain his love.
Elsewhere in Athens, a group of manual laborers discuss a play, Pyramus and Thisbe that they hope to perform at the Duke's wedding. Bottom, a weaver with many ridiculous opinions about acting, gets the part of Pyramus. The group agrees to rehearse in the forest outside Athens. Meanwhile, in the forest, Oberon, the king of the Fairies, is fighting with his queen, Titania, over possession of a beautiful Indian changeling boy. Oberon decides to punish his wife for refusing to obey him. He sends his servant, Robin Goodfellow, a mischievous fairy also known as Puck, to bring him the love-in-idleness flower. The magical juice from this flower causes a person (or fairy) to fall in love with the first thing he or she sees. Just then, Oberon sees Helena following Demetrius through the forest and hears him threaten to abandon her. Oberon decides to make Demetrius love Helena and tells Puck to put the love juice on the eyes of the man in Athenian clothes. Oberon then sneaks up to the sleeping Titania and drops the potion on her eyes.
As all this is going on, Lysander and Hermia get lost in the forest, and find a place to sleep, apart, for the night. Puck sees Lysander's Athenian clothes, and puts the love juice on his eyelids. Nearby, Demetrius finally abandons Helena. Lysander wakes, sees Helena, and falls in love. Helena thinks Lysander is mocking her with his declarations of love, and stalks off. Lysander follows. A moment later, Hermia wakes up. Shocked that Lysander would abandon her, she goes to search for him.
The laborers rehearse their play in the forest, close to where Titania sleeps. They can't remember their lines or cues, amusing Puck, who's watching them. Puck transforms Bottom's head into the head of an ass; the other laborers, terrified, run away. Not knowing what's happened, and thinking his friends ran away as a joke, Bottom sings to show he isn't frightened. His song wakes up Titania. She falls instantly in love.
Oberon is pleased with Puck's work, until he discovers that Puck put the love juice in Lysander's rather than Demetrius's eyes. He sends Puck to bring Helena to Demetrius, and puts the love potion in Demetrius's eyes. Helena arrives, followed by Lysander. Demetrius wakes, and falls in love. Both men argue over who deserves Helena, while she now thinks they're both mocking her. Hermia then shows up, and is furious that Lysander is now wooing Helena. Soon Hermia and Helena are on the verge of fighting. When the men go off to duel in the forest, Helena runs. Hermia chases her. Oberon sends Puck out to make sure no one hurts each other. Puck uses his trickery to get them all to fall asleep in the same small glade, and then puts the love juice on Lysander's eyelids so he'll love Hermia again. Titania, meanwhile, is still doting on Bottom, and has given Oberon the changeling. While she sleeps, Oberon removes the spell, and Puck removes Bottom's ass-head.
On a hunting expedition in the forest, Theseus, Hippolyta and Egeus encounter the four sleeping lovers. When Lysander admits that he and Hermia were eloping, Egeus insists that the law be brought down on their heads. But Theseus sees that Lysander now loves Hermia while Demetrius loves Helena, and says the four lovers can marry at his wedding. At the same time, Bottom returns to Athens just as the laborers were starting to despair they wouldn't be able to perform their play because they had no Pyramus.
As entertainment after the wedding, the Duke chooses to watch the laborer's play. It is terrible and ridiculous, but the Duke and lovers enjoy making fun of it. After the three married couples go to bed, the Fairies enter and bless the marriages.