After Hermia has confided in Helena regarding her plans to elope with Lysander, Helena reveals the plan to Demetrius, in the hopes of procuring Demetrius's trust. However, Demetrius merely goes to the forest seeking Hermia, without giving Helena a second thought. After that he returns to the king.
Helena follows Demetrius, and Oberon, the fairy king, sees them arguing. Oberon feels pity on Helena and decides to help her by putting love juice on Demetrius's eyes, thereby compelling Demetrius to return Helena's love. Oberon instructs Puck, another fairy, to pour love juice on the eyelids of the "Athenian man"
However, Puck sees Lysander sleeping, and pours the love juice in Lysander's eyes instead, thus causing Lysander to fall in love with Helena (and abandon Hermia), while Demetrius's love for Hermia continues unaltered. Later, Puck pours the love juice in Demetrius' eyes as well, with the result of both Demetrius and Lysander falling in love with Helena and despising Hermia. They fight over Helena, until Puck lulls them to sleep, and then Puck reverses the spell upon Lysander so that Lysander will love Hermia again. The spell on Demetrius, however, is not removed, and the play ends with Demetrius very much in love with Helena.
Demetrius has to marry Hermia, while Hermia wants to marry Lysander. When Puck corrects his mistake and Demetrius then falls for Helena, it is because of the love potion.
After being found sleeping in the woods by the duke and duchess, Demetrius confesses his love for Helena while denouncing his earlier infatuation with Hermia. Gaining approval from Theseus, the four lovers take their first step towards their happy ending. Demetrius then watches the tale of Pyramids and This be performed by the six craftsmen on Duke Theseus and the Duchess's wedding at night, alongside Helena, Lysander and Hermia. Demetrius and the other four lovers share a wedding with Theseus and his soon-to-be wife.
Demetrius is a victim who got cursed by the spell of a mischievous fairy called Puck the spell made Demetrius fall in love with Hermia.