Alcestis was the eldest daughter of Pelias, king of Iolcus. (Cf. the story of Jason and Medea.) Her story has two main episodes: her marriage to Admetus, king of Pherae (in Thessaly), after he meets the test set by her father (to yoke a lion and boar to a chariot); her voluntary dying in place of her husband and Persephone or Heracles rescuing her from the underworld. Her voluntary death and her rescue both have a background in folklore.
The play tells of the bride who, on her wedding day, agrees to sacrifice her life for her husband if necessary, and then, after years of happy marriage, is held to her promise. Albin Lesky did the fundamental comparative study. He showed that the basic motif is the wife's sacrifice of herself for her husband (T211.1 in The Motif-Index), which is the kernel of the type (899 in The Types of the Folktale). (The motif is variable: the husband or fiancé may sacrifice himself for the woman.) Lesky showed that the refusal of the protagonist's mother or father (again there is variation within the motif) to die for him belongs to the type. This motif is developed in the play into the debate between Admetus and his father.
With this type, Euripides combined another folklore motif, the contest with death. In the tragedy, Heracles, an old friend of Admetus, the husband of Alcestis, appears on the scene just after her death. When he learns what has happened, he goes to her grave, wrestles with Death, frees Alcestis, and restores her to her husband.
In the prologue of Euripidesí play, it emerges that Apollo was the one who, by tricking the Fates, arranged for Admetus to defer his death by means of a substitute. Why did Apollo want to help Admetus in this way? Once upon a time, Apollo offended Zeus and was punished with a term of servitude to a mortal, who was Admetus. Apollo found Admetus to be a kind master and remained grateful to him. How had Apollo offended Zeus? A conflict between the two gods arose over Asclepius, the son of Apollo. Asclepius used his medical knowledge to bring the dead back to life. Zeus was then angry and blasted Asclepius with his thunderbolt. Apollo then retaliated by killing the Cyclopes, who manufactured Zeusí thunderbolts. Zeus then retaliated against Apollo by sending him into servitude to a mortal.
Aarne-Thompson, The Types of the Folktale.
Stith Thompson, Motif-Index of Folklore (Copenhagen, 1957).