Milton opens by addressing not God or an angel but two sisters: "Voice, and Vers". He calls them a "Blest pair of Sirens", which is a daring choice, because in classical legend the Sirens lured sailors to their deaths with song. Milton rescues the word. His Sirens are "Sphear-born", children of the heavenly spheres, whose motion ancient thinkers imagined as music. Their job is to "wed" their "divine sounds", joining sung voice to the words of poetry, so that together they can "pierce" "dead things" with "inbreath'd sense". The verb is a forceful one. Hearing heavenly music, the poem suggests, takes a jolt, because our ordinary senses are numb.
What the pair is asked to conjure is the music of heaven, "that undisturbed Song of pure content". A note on the text matters here. Milton's manuscript reads "concent", an old word for harmony of voices, and the printed "content" is probably a misprint. The correction sharpens the poem's whole idea: heaven is where everything sounds in agreement.
The vision that follows is a single sweeping picture. The Seraphim stand "in burning row" blowing "Angel trumpets", the Cherubim sing "in thousand quires" and touch harps of "golden wires", and the "just Spirits" carry "victorious Palms". Milton lets the rhymes tumble out in couplets, "row" and "blow", "quires" and "wires", "Palms" and "Psalms", like a procession filing past. The sounds do the work too: "Saintly shout, and solemn Jubily" is full of hissing, ringing s and l sounds. Then, at the centre of all this noise, comes a hush. "To him that sits theron" is one of the shortest lines in the poem, and God is only "him", seen as a sapphire throne and nothing more. The poem is mostly in lines of ten syllables, with a few short ones breaking in. Those short lines work like pauses for breath, and "Singing everlastingly" then stretches out as long as the word itself.
Then the poem turns on "That we on Earth", and it becomes a story about loss. Humans once joined in the universal music "As once we did", until "disproportion'd sin / Jarr'd against natures chime". "Disproportion'd" is a musical idea as well as a moral one, since harmony depends on correct proportion between notes. Milton makes sin sound as ugly as it is. "Jarr'd" opens its line with a stressed thud, and "harsh din" is all clashing consonants. Against it he sets "Diapason", the octave, a word that rolls open like a chord. Creatures once answered their Lord because "love their motion sway'd", so that love set them moving, like dancers following a beat. "First obedience" suggests, judging by the wording, that Milton has the first fall of humankind in mind, but he never tells the story. He tells it only as a sound going out of tune.
The last four lines turn this diagnosis into a prayer. "O may we soon again renew that Song" is quick and personal, and "keep in tune with Heav'n" makes goodness sound almost like a daily practice. The final image is a "consort", the word for a group of musicians playing together, and the poem's closing line stretches to twelve syllables, longer than the ten-syllable lines around it: "To live with him, and sing in endles morn of light". The line is itself a small piece of endlessness, and its slow monosyllables make it feel unhurried.
The poem is significant because its form carries its argument. Twenty-four lines run on as one long sentence of longing before the closing prayer, and the poem reaches toward heaven in the way it describes, by wedding sound to sense. It also offers a startling way of thinking about right and wrong: to sin is to be out of tune, and to be saved is to be retuned. A reader who has ever winced at a wrong note will understand, instantly and physically, what Milton fears and what he hopes for.