That night in the woods, Titania's fairy followers sing her to sleep in a beautiful glade. Oberon then sneaks past the guard protecting her, and drops the juice on her sleeping eyelids. He hopes that when she wakes the first living thing she sees will be utterly vile, and exits.
Lysander and Hermia enter. They've gotten lost, and decide to spend the night where they are. Lysander wants them to sleep next to each other, but Hermia insists that they sleep apart in order to preserve her modesty until they're married. Lysander promises to obey her wishes, praying to die should he cease to be loyal.
For all his love, Lysander still tries to sleep with Hermia before they marry. For a woman, love is a threat. It can inspire her to pre-marital sex, which would cause her social ruin.
Once Hermia and Lysander fall asleep, Puck enters, complaining that he's searched the forest and hasn't found the Athenian youth he's looking for. Then he spots Lysander, and takes the fact that the two are sleeping far apart as proof that he is the man who was spurning the Athenian lady. He drops the potion on Lysander's eyes, and rushes back to Oberon.
Puck's error unleashes the love juice on Lysander. The audience can anticipate that Shakespeare will manage to get Lysander to see Helena when he wakes, and that hilarity will ensue.
Demetrius runs into the glade, pursued by Helena. He demands she cease following. She begs him to stay. But he runs on, and she's too out of breath to follow. Helena despairs, and concludes she must be ugly… but just then notices Lysander on the ground.
Helena's desperate sadness in love continues.
Helena wakes Lysander, who immediately professes his love for her. He curses Demetrius for mistreating her, and regrets all "the tedious minutes" he spent with Hermia now that he loves Helena. Helena thinks Lysander is mocking her. She exits. Lysander tells Hermia's sleeping form to never come near him again, and rushes after Helena.
From high school to Shakespeare's plays, a lover's greatest fear is that the person they love will cease to love them in favor of someone else. It's happened already to Helena. Now it happens to Hermia, though she doesn't know it yet.
Hermia suddenly wakes from a nightmare in which a serpent was eating her heart while Lysander stood by, smiling and doing nothing. When she discovers Lysander is gone, she is terrified, and goes to find him.
Hermia dreams that a snake (a symbol of betrayal) steals her heart (symbol of love). Well, she's half right. Her love has been stolen, but by magic not betrayal.