When the play begins, Lysander is in love with Hermia, but her father disapproves and instead insists that she marry Demetrius. Lysander and Hermia decide to run away, but they get lost in the woods. They decide to stop and rest for awhile. While they are sleeping, Puck comes along. He has been told to put a drop of love potion in Demetrius's eye so that he will fall in love with Helena. Puck has never seen Demetrius or Lysander; all he knows is that the man he is looking for will be dressed like an Athenian. When he sees Lysander, he believes that he's the man; so he puts the love potion in Lysander's eye. Unfortunately, it is Helena whom Lysander first sees when he wakes up, and because of the love potion, he falls for her. But Helena is in love with Demetrius!
So here's the irony: Lysander loves Hermia, but she is going to marry Demetrius. The potion makes Lysander fall in love with Helena, who wants to marry Demetrius. Whew!