In the first scene of Shakespeare's A Midsummer Night's Dream, Hermia's father, Egeus, appeals to Theseus, the duke of Athens, to order Hermia to marry Demetrius, who Hermia refuses to marry. Egeus prefers Demetrius over Lysander, the young man with whom Hermia is in love and actually wants to marry.
Lysander points out to Theseus that Demetrius was at one time in love with Hermia's best friend, Helena, but when Demetrius first saw Hermia, he immediately abandoned Helena.
LYSANDER. Demetrius, I'll avouch it to his head,
Made love to Nedar's daughter, Helena,
And won her soul; and she, sweet lady, dotes,
Devoutly dotes, dotes in idolatry,
Upon this spotted and inconstant man (1.1.108–112).
Later in the scene, Helena supports Lysander's story about Demetrius being a "spotted [morally compromised] and inconstant [unfaithful] man."
HELENA. For ere Demetrius look'd on Hermia's eyne,
He hail'd down oaths that he was only mine;
And when this hail some heat from Hermia felt,
So he dissolv'd, and showers of oaths did melt (1.1.247–250).
Theseus orders Hermia to marry Demetrius, and Hermia and Lysander decide to run away to be married outside Theseus's jurisdiction and beyond Egeus's influence. They agree to meet in the woods outside Athens the next day. Lysander and Hermia confide their plan to Helena, who then tells the plan to Demetrius, hoping that she can prevail on Demetrius to love her instead of Hermia.
Demetrius follows Hermia into the woods, where he reveals another unpleasant side of his personality. Helena follows Demetrius into the woods, and when they meet, Demetrius is verbally abusive to her and threatens to abandon her to the wild animals in the woods, or do worse to her.
DEMETRIUS. I love thee not, therefore pursue me not....
Hence, get thee gone, and follow me no more. ...
Tempt not too much the hatred of my spirit;
For I am sick when I do look on thee....
I'll run from thee and hide me in the brakes,
And leave thee to the mercy of wild beasts. ...
Or, if thou follow me, do not believe
But I shall do thee mischief in the wood. (2.1.192, 198, 215–216, 231–232, 240–241)
Oberon, the king of the fairies, secretly observes the way that Demetrius treats Helena, and finds him "a disdainful youth" (2.1.266). These are strong words from Oberon, who is no paragon of sensitivity himself.
Oberon tells Puck to take some of the love potion that he intends to use on Titania, the queen of the fairies, find Demetrius, and "anoint his [Demetrius's] eyes" with it, so that " he may prove / More fond on her than she upon her love" (2.1.270–271).
Puck does as he's told, and with the help of the love potion, Demetrius falls hopelessly in love with Helena and now disdains Hermia.
After some further merry mix-ups with the love potion and much running around in the woods, Lysander, Hermia, Helena, and Demetrius eventually find love with the right person, the couples are married, and they all live happily ever after.
What remains to be seen is whether Demetrius remains true to Helena as he promises or returns to his "spotted and inconstant" ways.