Titania bids her fairies sing her a lullaby, which sends her asleep. The fairies depart, and Oberon enters and squeezes the juice on Titania's eyelids. He tells her to awaken when some vile thing is near.
Lysander and Hermia enter. They are lost, and decide to rest. Hermia insists that Lysander lie down some distance away from her, to ensure her chastity. They both sleep. Puck enters, and mistakes Lysander for Demetrius. He puts some of the flower-juice on Lysander's eyelids.
Demetrius enters, with Helena chasing him. He tells her to leave him alone, and goes off on his own. Helena comes upon the sleeping Lysander and wakes him up. As soon as Lysander opens his eyes he falls wildly in love with Helena and wants to kill Demetrius. He regrets all the time he spent with Hermia; Helena is much more worthy of his love. Helena, bewildered, can only assume that Lysander is making fun of her, and protests that she does not deserve to be mocked. She exits, upset and confused.
Lysander turns to the sleeping Hermia and says that he now loathes her, and has eyes only for Helena. He goes off to find her.
Hermia wakes with a start, calling for Lysander to help her. She says she dreamt a serpent was eating away at her heart, while Lysander sat by smiling. On finding that Lysander has gone, she is terrified at being alone in the wood.
The love-juice does its work, further revealing the irrationality of love. It is a fickle emotion and can change at any moment.