Euripides's Ion was first performed in the approximate year of 412 BCE. This means that it was produced in the middle to later part of Euripides's career as a playwright (after the famous Medea and before the equally famous Bacchae). The play tells the story of Apollo's son, Ion (in alternate versions, he is Xuthus's biological son). In Euripides's version, the god Hermes narrates the backstory to the audience at the opening of the play, which is as follows: Phoebus Apollo had an affair with the mortal woman, Creusa, who bore a son, Ion, in secret. Creusa, fearing the punishment of her father, King Erectheus, left the son in a basket in a cave, expecting he would die.

The action of the play picks up with Ion (whose name happens to be Greek for "going") sweeping the temple of Apollo at Delphi, home of a famous oracle. Creusa and her mortal husband, Xuthus, have come to pray for the birth of a child. While Xuthus is inside the chamber consulting the oracle, Creusa (as a ruse) describes to Ion the circumstances of "her friend" who gave birth to a son by Apollo but was forced to expose him. In smiler fashion, Ion admits that he was raised in the temple as an orphan, and he longs to know his mother.

After consulting the oracle, Xuthus immediately adopts Ion as a son, advised by the oracle that the next person he meets upon exiting the shrine is his son. Creusa is angry because she assumes that Ion is Xuthus's son by a slave woman. She plots to have a tutor poison Ion, but the plot is revealed when a bird accidentally drinks his poisoned wine at a banquet.

Creusa is incriminated by the tutor but realizes that Ion is her son when she sees the basket in which she exposed him and which Ion brought to Athens from the temple where the priestess of Apollo gave it to him. Once she recognizes him, Creusa embraces Ion as her son. She has to convince him by means of identifying the contents of the basket without looking inside (a weaving of a Gorgon's head, a pair of serpents, and an olive branch). After the revelation, Athena appears and verifies that Apollo (and not Xuthus) is in fact Ion's father, but they all agree to keep the secret from Xuthus.