Ion, the son of Phoebus Apollo and Creusa, does not know his origins. Because he serves Apollo, Ion is deeply devoted to him. He lavishly praises the god, his own efforts, and the efforts of the other temple servants on his behalf. Ion considers Phoebus his spiritual father but does not know that he is his biological father:

ION: Fair is the task wherewith I minister before your house, O Phoebus, and honor your oracular habitation. Glorious is my task, I tender the hand of service to gods, not to mortals but immortals. To toil at labors so blessed I weary not. . . . I call my benefactor the title of father, even Phoebus, the temple's lord.

While serving at Apollo's temple, he meets Creusa when she and her husband, Xuthus, arrive to seek counsel about their own childless state. When Creusa tells a story about having borne Apollo's baby, only to have it taken away, she attributes this experience to another woman. Ion is shocked and incredulous that she speaks this way of Apollo:

ION: . . . I know neither mother nor father.

. . .

CREUSA: Unhappy is your mother! Whoever was she?

ION: Perhaps my mother is some wronged woman. . . .

CREUSA: Ah! I know another woman who was wronged as your mother was. . . . Phoebus lay with her—so says this friend of mine.

ION: A woman? With Phoebus? Stranger, do not say such a thing.

As they continue to speak and are struck by the similarities of their situations, they wonder if the "friend's" baby even survived. Ion advises Creusa to speak carefully lest she bring down the god's wrath. Meanwhile, Xuthus has been consulting the oracle, which has mysteriously proclaimed a child's arrival.

The importance of children and lineage as themes of the whole play is emphasized by the Chorus, which offers extended praise of progeny. Children carry on the family line, and are themselves rewarded by inheriting wealth and talents. The Chorus likewise stresses the emotional benefits of having children, both in good times and bad:

CHORUS: They are our strength in time of trouble, and in happiness our delight; in war they are a tower of strength to preserve their native land. Far dearer than wealth, far dearer than princely halls to me are children to care for, children to rear in virtue.

When Ion and Xuthus meet, Creusa is not present. Xuthus is exuberant in his greeting, rushing to embrace the youth, who is both mystified and irritated at his familiarity. He threatens to shoot the older man with an arrow unless he backs off:

XUTHUS: Give me your hand's dear greeting. Let me embrace you.

ION: Are you in your right mind? Has some god-sent stroke demented you?

XUTHUS: I am in my senses. I have found my nearest and dearest and I do not want you to get away.

The oracle has given Xuthus the prophecy that, after leaving the cave, the first person he beholds will be his child. That person is Ion. The two of them next try to figure out how Xuthus could be his father. Xuthus finally admits to having had a dalliance during a festival to Bacchus with timing that matches Ion's begetting. This means that the identity of the mother is unknown. Ion accepts that Xuthus is his father but still yearns to know his mother:

ION: Ah, dear mother, when will I see your face? I yearn to see you more than ever, whoever you are. Perhaps you are dead, and I shall never be able to.