Oberon got very mad at Titania so he asked Puck to make a potion that would make Titania fall in love with the first person she sees.
Nearby there was a rehearsal for the play being performed for a wedding. Bottom did not agree with what was happening and stormed of into the woods. On his way puck gave him the head of a donkey, Titania awoke and fell istantly in love with bottom.
Oberon noticed Lysander, Hermia, Helena and Demetrius arguing so he asked Puck to pour more of the potion on to each one so that they would fall in love with the correct person. But it didn't go to plan, Lysander and Demetrius both fell in love with Helena.