It follows the orphan Ion, a young and willing servant in Apollo's temple, as he inadvertently discovers his biological origins.

Euripides' retelling of this myth is a radical step forward among the Greek tragedies: while in other plays of classical Athens individuals often rail against the disasters that the Fates or the gods have caused to befall them, in this powerful play both Creusa and Ion actually challenge whether the gods have any right to govern the destinies of human beings.

The god Apollo raped her in a cave; there she secretly gave birth to his son whom she intended to kill by exposure to the elements for fear of the terrible punishment she would receive at the hands of her parents for having a child apparently out of wedlock.

Many years later, near the end of childbearing age and so far unable to have a child with her husband Xuthus, a Thessalian and son of Aeolus, they traveled to Delphi to seek a sign from the oracles regarding whether there was any hope of their having any children in the future.

Apollo concealed her pregnancy from her father and Creusa secretly gave birth to a child, whom she left in a basket, along with some sacred trinkets to protect him, expecting that he might be devoured by beasts.

However, when she returns to find him he has disappeared because, unknown to her, Apollo has sent Hermes to bring the boy to Delphi where he has grown up as an attendant at the temple.

As Ion sweeps the steps of the temple with a broom of laurel, he sings the praise of the god who is like a father to him.

His reveries are disturbed by birds which he shoos away with his arrows in order to keep the sacred temple clean, though not without a twinge of regret as he also sees them as being connected to the gods.

The Chorus, consisting of Athenian maidens, arrives at the temple and marvels at the stonework depicting ancient legends.

They identify themselves to Ion as servants of the Athenian rulers and soon spot their mistress arriving at the temple doors.

Moved by the thought of his mother, Creusa tells Ion that she has come in advance of her husband to question the oracle on behalf of "a friend" who had a child by Apollo, which she abandoned.

While Xuthus is inside, the Chorus of Creusa's servants prays to Athena and Artemis, recalling the joys of fertility and raising children.

Recalling the story of the daughters of Cecrops and Aglauros, they conclude that children born of mortals by gods are fated for ill-fortune.

Xuthus proposes that Ion come back to Athens with him, but the young man is reluctant to take on the role of "the bastard son of an imported father."

As he leaves to offer sacrifice, he names the boy Ion because he met him 'coming out' and tells him to arrange for a banquet to celebrate his departure from Delphi.

Ion reluctantly agrees to go to Athens, but he longs to meet his unknown mother and fears he will not be well received.

The old tutor speculates that Xuthus discovered that Creusa was barren, sired the child by a slave, and gave him to a Delphian to raise.

He tells them (in a typically Euripidean messenger speech) that a Delphian mob is searching for Creusa to stone her to death.

As Ion rails against the laws that protect convicted assassins, the Pythian priestess emerges from the temple.

When Ion questions her about his father, Creusa tells him with some embarrassment that he is the son of Apollo and that she reluctantly abandoned him in a deserted cave to be the prey of birds.

As they celebrate their change of fortune, Ion takes her aside to ask if perhaps she conceived him with a mortal father and made up the story about Apollo.

Convinced that only Apollo can tell him for certain who his father is, Ion starts toward the sanctuary to confront the god, but he is stopped by the appearance of the goddess Athena on the roof of the temple (an instance of deus ex machina).

Athena then tells Creusa to establish Ion on the ancient Athenian throne where he will be famous throughout Hellas.

He and his half brothers will establish the Ionian, Dorian, and Achaean races of Greeks (thus going from being an orphan to ultimately becoming a patriarch of his people).

In The Classical Quarterly, Spencer Cole defended another scholar's argument that the play is "self-referential to a degree unparalleled anywhere else in Euripides," and wrote that Ion was the work in which the tragedian's will to innovate was most evident.