Ion (/ˈaɪɒn/; Ancient Greek: Ἴων, Iōn) is an ancient Greek play by Euripides, thought to have been written between 414 and 412 BC.^{[citation needed]} It follows the orphan Ion, a young and willing servant in Apollo's temple, as he inadvertently discovers his biological origins. As it unfolds the play is also the powerful story of his mother, Creusa, as she strives to guide her own life after having experienced terrible abuse at the hands of a god who is beyond her power (or that of any mortal). 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. In the end, however, Euripides takes a step back from this precipitous development in human thought.

Creusa, daughter of Erechtheus, was a noble native of Athens and daughter of the king. 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. Throughout her life only she knows of this secret. 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.

Outside the temple of Apollo at Delphi, Hermes recalls the time when Creusa, the daughter of Erectheus, was raped by Apollo in a cave at Long Rocks under the Acropolis. 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.

Creusa, meanwhile, was married to the foreign-born Xuthus, son of Aeolus, the son of Zeus. Xuthus won Creusa by assisting the Athenians in a war against the Chalcidians. Xuthus and Creusa have come to Delphi to ask if they can have children. Hermes says that Apollo will give the boy, soon to be named Ion, to Xuthus who will take him home to Athens where he will be recognized by his mother.

Hermes steps into a wooded grove when Ion arrives to begin his morning chores. 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.

Creusa introduces herself to Ion as the daughter of Erectheus. Ion is impressed, as he is familiar with the old stories about her family. Ion's casual mention of Long Rocks startles Creusa but she reveals nothing of her past. She tells him that she has married a foreigner, Xuthus, who won her as a prize for helping the Athenians in battle. They are here to ask about having children. Ion introduces himself as an orphan slave who was brought up by the priestess of Apollo. When Creusa asks if he has ever tried to find his mother, he says he has no token of her. 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. She has come, she tells him, to ask the god if her friend's child is still alive. He would be about your age now, she tells him. Ion warns her to abandon the inquiry, saying that no one would dare accuse the god of such a deed in his own temple. Seeing Xuthus approaching, Creusa asks Ion to reveal nothing of their conversation. Xuthus arrives and expresses confidence that he will receive good news from the oracle. He sends Creusa with laurel branches to make the rounds of the external altars and goes into the sanctuary. After they both leave, Ion questions how the gods, who punish evildoing among mortals, can engage in abusive behavior themselves. Before going off to finish his chores, he indignantly advises the gods not to sleep with mortal women just because they can.

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.