1.1: In Athens, Demetrius and Egeus appear before Duke Theseus. We learn that Demetrius and Hermia are engaged (thanks to Hermia's dad), but Hermia doesn't want to marry him—she wants to marry her boyfriend, Lysander.
1.1: We learn that Demetrius was once engaged to Helena, who is still in love with him.
2.1: Demetrius shows up in the wood looking for Lysander and Hermia, who have run off to elope. Demetrius is looking to kill Lysander.
2.1: Demetrius is followed by Helena, who begs him to love her again. He treats her like garbage and tells her to scram... or else.
2.1: Demetrius runs off to hide in the bushes, so Helena will be left "to the mercy of wild beasts."
3.2: Demetrius finds Hermia in the wood and begs her to love him.
3.2: Demetrius is accused by Hermia of killing Lysander, who is nowhere to be found. (Demetrius didn't kill him. Lysander has been dosed with Oberon's magic love juice and is busy chasing Helena around the wood, but Demetrius doesn't know about this.)
3.2: Demetrius argues with Hermia, which makes him sleepy. He takes a nap.
3.2: Demetrius is dosed by Puck with the magic love juice. When he wakes up, he sees Helena and falls in love with her. (Hermia who?)
3.2: Demetrius and Lysander fight over Helena and try to convince her that they really do love her.
3.2: Demetrius and Lysander run off to duke it out, man to man.
3.3: Demetrius gets tired and falls asleep after trying to find Lysander in the wood.
4.1: Demetrius wakes up next to the other Athenians the following morning. He's says groggily that he can't believe what happened the night before.
4.1: Demetrius tells Duke Theseus that he no longer loves Hermia. Now, he wants to marry Helena.
4.1: Off-stage, Demetrius marries Helena in a triple wedding ceremony that also includes Hermia and Lysander and Theseus and Hippolyta.
5.1: Demetrius watches the Mechanicals perform a play and makes snarky comments throughout the silly performance of Pyramus and Thisbe.
5.1: After the play, Demetrius goes to bed with Helena.