Rather than attempt my own summary of John Milton’s poem, I give selected excerpts from his original, with my comments given, where necessary, in italics. Excerpts given as prose are from Milton’s own summaries at the start of each book. Those given in verse are taken from the body of the poem. Milton is a superb wordsmith, and I have only adjusted a few of his spellings to their modern forms.

This First Book proposes, first in brief, the whole subject, Man’s disobedience, and the loss thereupon of Paradise wherein he was placed: then touches the prime cause of his fall, the Serpent, or rather Satan in the serpent; who revolting from God, and drawing to his side many legions of Angels, was by the command of God driven out of Heaven, with all his crew, into the great Deep.

… he it was, whose guile,
Stirred up with envy and revenge, deceived
The mother of mankind, what time his pride
Had cast him out from Heaven, with all his host
Of rebel Angels, by whose aid, aspiring
To set himself in glory above his peers,
He trusted to have equalled the Most High,
If he opposed; and with ambitious aim
Against the throne and monarchy of God
Raised impious war in Heaven and battle proud,
With vain attempt. Him the almighty Power
Hurled headlong flaming from the ethereal sky.
With hideous ruin and combustion, down
To bottomless perdition; there to dwells
In Adamantine chains and penal fire,
Who durst defy the omnipotent to arms.

Which action passed over, the poem hastens into the midst of things, presenting Satan with his Angels now fallen into Hell, described here, not in the Centre, (for heaven and earth may be supposed as yet not made, certainly not yet accursed,) but in a place of utter darkness, fitliest called Chaos: here Satan, with his Angels lying on the burning lake, thunder-struck and astonished, after a certain space recovers, as from confusion, calls up him who next in order and dignity lay by him; they confer of their miserable fall.

So stretched out huge in length the Arch-Fiend lay,
Chained on the burning lake, nor even thence
Had risen or heaved his head, but that the will
And high permission of all-ruling Heaven
Left him at large to his own dark designs,
That with reiterated crimes he might
Heap on himself damnation, while he sought
Evil to others, and enraged might see
How all his malice served but to bring forth
Infinite goodness, grace and mercy shewn
On Man by him seduced, but on himself
Treble confusion, wrath and vengeance poured
Forthwith upright he rears from off the pool
His mighty stature. On each hand the flames
Driven backward slope their pointing spires, and, rolled
In billows, leave in the midst a horrid vale.

Satan awakens all his legions, who lay till then in the same manner confounded; they rise, their numbers, array of battle, their chief leaders named, according to the idols known afterwards in Canaan and the countries adjoining.

“Awake! arise! or be for ever fallen.”
They heard and were abashed, and up they sprung
Upon the wing; as when men wont to watch,
On duty sleeping found by whom they dread,
Rouse and bestir themselves ere well awake.
Nor did they not perceive the evil plight
In which they were, or the fierce pains not feel;
Yet to their general’s voice they soon obeyed,
Innumerable. As when the potent rod
Of Amram’s son, in Egypt’s evil day,
Waved round the coast, up-called a pitchy cloud
Of locusts, warping on the eastern wind,
That o’er the realm of impious Pharaoh hung
Like night, and darkened all the land of Nile:
So numberless were those bad Angels seen,
Hovering on wing under the cope of Hell,
‘Twixt upper, nether, and surrounding fires;

Meanwhile the winged heralds, by command
Of sovereign power, with awful ceremony
And trumpet’s sound, throughout the host proclaim
A solemn council forthwith to be held
At Pandemonium, the high capital
Of Satan and his peers. Their summons called,
From every band and squared regiment,
By place or choice the worthiest; they anon
With hundreds and with thousands trooping came
Attended.

To these Satan directs his speech, comforts them with hopes yet of regaining Heaven, but tells them lastly of a new world and new kind of creature to be created, according to an ancient prophecy or report in Heaven; for that Angels were long before this visible creation, was the opinion of many ancient Fathers. To find out the truth of this prophecy, and what to determine on, he refers to a full council. What his associates thence attempt. Pandemonium the palace of Satan rises, suddenly built out of the Deep; the infernal Peers there sit in council.

Source of text: Wikisource.

References

Wikipedia on John Milton
Wikipedia on Paradise Lost
Wikimedia text of Paradise Lost

Dartmouth’s superb annotated version in its John Milton Reading Room.

John Leonard (ed) (2000) Paradise Lost, John Milton, Penguin Classics. ISBN 978 0 140 42439 3.
Gordon Teskey (ed) (2005) Paradise Lost, John Milton, Norton Critical Editions. ISBN 978 0 393 92428 2.
Louis Schwartz (ed) (2014) The Cambridge Companion to Paradise Lost, Cambridge UP. ISBN 978 1 107 02946 0.