Bottom’s head is replaced with an ass’s head by Puck.
The craftsmen decide to rehearse their play in the woods because they are getting harassed in town. The woods rehearsal is fine, but Puck decides to watch the “hempen home-spuns” because they are in fairy territory and very close to where Titania is sleeping.
As part of the rehearsal, Bottom goes backstage to another part of the forest. Puck knows that Titania has been anointed because Oberon wants her to fall in love with a beast, so he kills two birds with one stone and replaces Bottom’s head with the head of a donkey.
When Bottom says his line, Quince and the others scream and panic.
O monstrous! O strange! We are haunted. Pray,
masters! fly, masters! Help! (Act 3, Scene 1)
Bottom thinks they are making fun of him, and gets upset. He starts singing, and wakes Titania. She falls in love with him, because she is under the power of the spell, and he still does not know he has an ass’s head.
Bottom remains with Titania for a bit, and is waited on by her servants. He falls asleep, and Oberon frees Titania of the curse. She is repulsed by Bottom and leaves, so that he wakes up alone and confused.