The Metamorphoses opens with chaos resolving into an ordered world and closes with the soul of Julius Caesar carried to the stars, and in between it moves without a break through the whole body of Greek and Roman myth. The organising device is transformation: Daphne into laurel, Actaeon into a stag, Arachne into a spider, Narcissus into a flower, Philomela into a bird, Pygmalion's statue into a woman. Roughly 250 such episodes are strung together, sometimes by chronology, sometimes by geography, sometimes by a character in one tale beginning to narrate the next, so that the poem runs continuously from first line to last with no separable units.
The arrangement is loosely historical: books one to two are cosmic and divine, the middle books largely heroic, Perseus, Medea, Theseus, the Calydonian boar, Hercules, and the last books move into the Trojan war, Aeneas, the Roman kings and finally Augustan Rome. The final book gives long space to Pythagoras expounding the transmigration of souls and the flux of all things, which supplies a philosophical gloss on everything preceding, though how seriously Ovid intends it is disputed.
Tone shifts constantly. The rapes are frequent and, read attentively, not comic; the divine punishments are disproportionate; the gods are vain and dangerous. Modern criticism has moved a long way from treating the poem as a charming anthology.
For much of Greek and Roman myth the Metamorphoses is not merely the best-known source but the only full one. Stories that existed in scattered Hellenistic poetry and local tradition survive as continuous narrative because Ovid retold them, and where he invented, elaborated or combined variants there is often nothing left to check him against. That makes the poem indispensable and, as evidence for what Greeks and Romans actually believed, unreliable in a specific way: it is a first-century literary reworking, not a record of cult.