The play deals with the best known episode from Alcestis' life: her death. Apollo comes out from Admetus' palace and tells the audience how he was punished by Zeus to be the king's servant. Because Admetus had been kind to him, Apollo made the Fates/Moirai promise to let Admetus live beyond his allotted time, if he finds someone willing to die instead of him. When the time of his death came, he asked his friends to take his place, then his father and his mother, but they all turned him down. The only one who volunteered to die was his wife, Alcestis, who didn't want to remain without her beloved husband and who also didn't want their children to remain without a father.
In the meantime, Thanatos approaches the palace and is a little afraid when seeing Apollo, who used trickery to get that promise from the Fates: in fact, he gave them wine until they got drunk. Apollo tries to convince Thanatos to let Alcestis live, but he replies that Apollo can't have everything his way. Thanatos doesn't care to let her live long and die old and rich, because a young person is a richer prize for him. Furious, the god prophesyzes that someone will come (the hero Heracles), who is on his way to catch the man-eating mares of Diomedes, and he will wrestle Death and bring Alcestis back.
Thanatos goes into the palace, to cut a lock of hair with his sword (this ritual meant that the person would die). The Chorus comes in front of the palace and wonders: isn't this the day Alcestis was supposed to die? Why aren't there any signs of mourning - people beating their breasts, mourners' cries, bowls of lustral water in front of the doors? Have they took her to the tomb in secret? The Chorus also say that the only one who could bring her back to life was Apollo's son (i.e. Asclepius), but he was killed by the Zeus.
A teary-faced handmaid comes out of the palace. She tells them that Alcestis is alive and dead, that is, she is close to death. She also tells them that Admetus will realize his loss only after his wife is gone. The Chorus agrees that she is the best of women beneath the sun.
The maid tells them how Alcestis woke up, washed and dressed in her funeral robe, then prayed in front of the hearth fire (to goddess Hestia, probably) to be a good mother for her children, to find them good partners and to help them live a long and fulfilled life. Then she cried near her bridal bed, saying that she will die for the man she loves and that maybe, in that bed, another woman will sleep, maybe happier than her, but not truer in her love.
She bid farewell to her children and also to every slave in the house. None was too low, she had a good word for everyone. The maid adds that for Admetus it would have been easier to die, instead of living with this agony.
The Chorus prays to the gods to save Alcestis, when she comes out, together with her husband and children and followed by their servants.
Admetus prays the gods, too, while Alcestis is afraid, because she thinks she saw Charon, in a boat, waiting impatiently for her.
Before dying, she wants to tell him her wishes. She points out that his parents didn't want to sacrifice themselves for their son, and that she wouldn't have wanted to live her life as a widow with orphan children.
She asks her husband not to remarry, as a stepmother would be harsh with the children and especially to their daughter.
Admetus promises to mourn her all his life and he also decides to give up all the merrymaking, drinking and singing. He asks his wife to prepare the house where they will dwell together when he dies and he says he'll ask the children to bury him near his wife.
Admetus is so desperate, that he asks Alcestis to take him with her, but she answers that it is enough if she dies for him. She bids farewell to che children and dies, while Eumelus cries because of this tragedy.
The Chorus laments her death and promises to take care that Admetus should keep his promise to never remarry.
Right at this moment, Heracles appears. He was sent by king Eurystheus of Tiryns to bring the four-horsed chariot of Diomedes. The Chorus tells him that these horses are very dangerous, as they eat people.
When Admetus comes out, Heracles asks about the mourning. The king tells him he has "to bury a body", Heracles wants to know if it's someone from the family, and Admetus says they are all fine, while his wife "is and is not", i.e. she can't live when she agreed to die for him. Heracles misunderstands him and thinks that Alcestis is just sad because she has to die.
In the end, Admetus says that a woman is dead, who is not related to his house, but who lived there. Heracles offers to go and stay with someone else, because he doesn't want to disturb the mourning, but Admetus invites him to stay. He tells the serants not to siturb Heracles and he also tells the Chorus that he can't be inhospitable to a good friend.
The Chorus sings about the times when Apollo was the king's servant and they praise Admetus for being a good host. The funeral procession of Alcestis comes out of the house. Pheres, Admetus' father, praises her, saying that, without marriages such as this, it would be foolish to marry.