"Lamia" begins with the language of a fairy tale: "Upon a time."
Long, long ago, the speaker says, the woods were ruled not by "King Oberon" (the folkloric King of the Fairies), but by the nymphs, satyrs, fauns, and dryads of classical mythology—the strange, dangerous forest spirits who drank and danced and lusted under the trees. But then, the "faery broods" came along and took over, scaring away those older, wilder beings.
In these first lines, the speaker takes a curious position somewhere between storyteller and historian. In one sense, they're describing the timeless world of magic. In another, they're talking about epochs of legend as if they were dynasties: in the same way as the British monarchy passed from Tudors to Stuarts, the rule of the enchanted world passed from the Greek and Roman spirits to the fairies.
This oddly historical framing implies that this poem takes place not just in a world of different spirits, but of different beliefs. For in the time when nymphs and satyrs reigned, so did the Greek and Roman gods. The world the speaker describes is one in which the gods and goddesses themselves might descend from Mount Olympus (the sacred mountain that was their home) to meddle with human affairs. (One shortly will in this very poem.)
Fairies are tricksters, meddlers, tempters, enchanters—but gods, they are not. Looking back to the world of classical mythology, this speaker also looks back to a time when magic and belief were more tightly interwoven and far more powerful.
Back in this rich and perilous era, the speaker tells readers, the god Hermes once fell head over heels in love with a beautiful nymph. This was nothing unusual. Hermes (like a lot of the gods) was "ever-smitten," perpetually falling in love with someone or other. Nor was it unusual that he should descend from Mount Olympus looking to commit an "amorous theft"; as well as being the gods' messenger, Hermes was the tricksy patron of thieves. No wonder he should see the pursuit of love as a kind of burglary.
But perhaps it's a little bit more unusual that Hermes should fall prey to this passion without ever having laid eyes on this nymph himself. He falls in love not with the nymph, really, but with her reputation. All he knows is that, somewhere on the "shores of Crete" (a Greek island), there's a nymph so beautiful that the Tritons—male sea-spirits with fish tails—crawl on land to pour pearls at her feet, willing to "wither[]" away out of the water just to get a glimpse of her. In fact, all the woods of Crete are strewn with love-gifts this nymph's myriad suitors have left her.
Hearing rumors of these goings-on, Hermes finds himself inflamed. Take a closer look at Keats's imagery here:
Ah, what a world of love was at her feet!
So Hermes thought, and a celestial heat
Burnt from his winged heels to either ear,
That from a whiteness, as the lily clear,
Blush'd into roses 'mid his golden hair,
Fallen in jealous curls about his shoulders bare.
Passion, here, overtakes Hermes's entire divine body. On the one hand, this is a very elevated and "celestial" sort of seizure: he's all flower petals and gold. On the other, this is a vision of a god full-body blushing with desire, turning bright red, so overcome with his urge to have this unseen nymph for himself that his very hair becomes "jealous."
This image of a sudden, all-consuming bodily need (alongside the ominous vision of those poor languishing fish-out-of-water Tritons) introduces what will become one of the poem's central issues: the swift, dangerous power of passion. Hermes doesn't even have to see his nymph face to face to be overcome by desire for her. That sudden, unmanageable desire will lead to impetuous action and unforeseeable consequences—though not for Hermes himself.
Keats will tell the story of Hermes's passion and what became of it in heroic couplets: paired rhyming lines of iambic pentameter (that is, lines of five iambs, metrical feet with a da-DUM rhythm). Here's how that sounds in lines 13-14:
For some- | where in | that sa- | cred is- | land dwelt
A nymph, | to whom | all hoof- | ed Sa- | tyrs knelt;
(Note that, across "Lamia," many words ending in -ed take a two-syllable pronunciation: hoof-ed, charm-ed. Where Keats wants -ed words to be pronounced in one swoop, he often uses a contraction: ravish'd, blush'd.)
Keats will use this crisp, rigorous, traditional form—pioneered by Chaucer and refined by 17th- and 18th-century poets like Dryden and Pope—in an inventive and flexible way. Where the strictest heroic couplets are consistently end-stopped, Keats will use surprising enjambments (like the one in the lines quoted above). And where the Enlightenment-era poets used heroic couplets to write vicious satires or crystalline hymns to universal order, Keats will use them to tell a murky, mysterious, ambivalent tale of uncontrollable forces: dreams, magic, and passion.