Hermia and her boyfriend Lysander want to get married but Hermia's father doesn't approve. Hermia's father said she could either marry Demetrius or die because as her father he has that power. The King Theseus agrees to this decision.
Oberon the fairy king finds a love flower, he uses the flower juice to make Demetrius fall in love with Helena and not with Hermia that way Lysander and Hermia can continue to stay in love. But at first the helper fairy put the love juice in Lysanders' eyes so then he falls in love with Helena.
Since Lysander and Demetrius got the love juice put in their eyes and they both saw Helena first, now they love her. Helena thinks they are mocking her because she always follows Demetrius and they didn't like her at all before today. Oberon realized he messed up
You men are so rude tricking a lady like that!!
Oberon restores their memory before the love juice ever happened and then they officially become couples and are invited to get married at King Theseus' wedding.
Both couples get married, and so does King Theseus.