At the beginning of Shakespeare's A Midsummer Night's Dream, both Demetrius and Lysander are in love with the fair Hermia, and each seems ready to fight to claim her as his bride. Demetrius is actually more in favor with Hermia's father than is Lysander, but it is Lysander that Hermia loves. In the meantime, we are introduced to Helena, who loves Demetrius passionately - to the point that she would do anything for him. Demetrius has been involved with Helena, but he previously dumped her in his pursuit of Hermia. This does not stop Helena from still pursuing Demetrius to win him back.
When Lysander and Hermia run away, Demetrius pursues Hermia, and Helena pursues Demetrius. When they are in the forest at night, Oberon, King of the Faeries, takes pity on Helena and causes Demetrius to fall in love with her. One side effect of this is that Oberon's henchman, Puck, screws up and causes Lysander to fall in love with Helena, too. However, it all gets sorted out, and at the end of the play, Demetrius and Helena are one of the three happy couples who marry.
Demetrius is at first in love with beautiful Hermia. However, so is Lysander. Hermia is in love with Lysander and Helena is in love with Demetrius. Lysander and Hermia elope to the forests. Hermia follows them in the hope that she can impress Demetrius by being of use to him. Demetrius follows the eloping couple in the forests and Helena follows him. Puck accidentally places the love juice potion on Lysander's eyes and causes him to fall in love with Helena. To right this wrong, Puck tries again and bungles it up by causing Demetrius to also love Helena. Helena accuses Hermia of putting the two men up to this facade to torment her. Eventually, all is sorted out and Demetrius comes out of the fiasco loving Helena.