Demetrius loves Hermia - he wants to marry her, and her father agrees - he asked the Duke of Athens to make her obey her father, but Hermia plans to run away with Lysander. Hermia tells Helena about her plan.
Demetrius heads into the forest to find Helena and Lysander, followed by Helena - everyone gets lost! Oberon and Titania (King & Queen of the Fairies) have argued. Oberon sees Demetrius & Helena arguing too so sends his servant Puck for a magic potion which is sprinkled into their sleeping eyes. They will fall in love when they awake and see each other.
Adjusting the magic!
Lysander sees and falls in love with Helena - she thinks he is joking and is annoyed because she knows he loves Hermia. Hermia is annoyed because Demetrius and Lysander used to love her but now only Demetrius does. Puck finds it hilarious.
Oberon continues the mischief, getting Puck to turn the workman called Bottom's head into the head of a donkey. Titania awakes from her tampered sleep, falling in love with Bottom as soon as she sees him.
Puck puts everything right again - Oberon & Titania return to their forest home, Helena & Demetrius get married, Hermia & Lysander get married, the marriage of the Duke of Athens goes ahead as planned and Bottom performs his play at the wedding celebration.