The poem opens with an invocation; that's when the speaker asks the muses – ancient deities thought to inspire poetry and art – to inspire him, give him the ability to perform, etc. We see speakers talk to their muses in the beginning of a lot of epic poems; check out the first lines of the Iliad.
He asks the muses to sing about "man's first disobedience" (1), the Forbidden Fruit, his exile from Eden, his eventual redemption through Jesus Christ, etc.
Soon, the scene shifts to a burning inferno; we're in Hell with Satan, only Hell isn't below the earth but somewhere way out in the middle of nowhere, a place Milton calls Chaos.
Milton's universe is tricky, so we'll give you a quick lay of the land. Basically, the created universe (the earth, the sun, the planets, the stars, etc.) is an enclosed globe or spherical structure. This structure hangs from Heaven by a golden chain. Everything outside the sphere and Heaven is called Chaos, with Hell at the end opposite to Heaven and the universe. Head over to "Best of the Web" to see some pictures.
Satan looks around bewildered; apparently he's just fallen from Heaven and hasn't quite adjusted to his new surroundings. It's hot, and there's a weird "darkness visible" all around.
He notices his first mate, Beelzebub.
Satan addresses Beelzebub, saying he doesn't look like the friend he knew in Heaven (apparently, the fallen angels have also undergone a change in appearance as well as location).
Satan describes how he and a bunch of other angels fought with God and lost. Although they've been beaten, all is not lost.
Beelzebub responds, saying that he's upset and worried about the current state of affairs.
He suggests that the only reason they still feel strong and courageous – still feel alive – is so that they can completely experience their punishment and satisfy God's "vengeful ire."
Satan responds, saying that their goal from now on is to be evil: "To do ought good never will be our task, / But ever to do ill our sole delight" (1.159-60). If God does something good, they will try to screw it up.
Satan suggests that he and Beelzebub move to a nearby plain and think about how to war against God, deal with the horrors of their circumstances, and repair their losses.
As Satan moves towards the plain, the narrator describes him: he is much bigger than any of the famous giants of classical mythology or the bible. He is so big, a sailor might mistake him for an island and attempt to moor his boat there.
He moves off the lake and flies – these fallen angels still have their wings – to the plain, which is also burning. Beelzebub eventually follows him.
Satan looks around and says it's not so bad because he'd rather be as far from God as possible.
He then suggests that his forces reassemble on the plain so they can figure out a plan of action.
Satan goes to the shore of the burning lake to beckon the fallen angels; his shield is almost as big as the moon and his spear is much bigger than the biggest mast of a ship.
The fallen angels are scattered on the lake like a whole bunch of leaves, or just like a whole bunch of reeds in the Red Sea.
Satan addresses the fallen angels, and he can't believe they've been vanquished.
He tells them to rise up now, or remain fallen forever. They rise up very quickly, as if they've been caught napping while on duty (that's Milton's comparison not ours!).
The angels assemble in squadrons, just like an organized army. There are a ton of them! The leaders of the squadrons assemble close to Satan, the "great commander."
These leaders will eventually become the various pagan deities described in the Old Testament (the first half of the Bible that deals with the times before Jesus) that the Israelites worshipped (sinfully) alongside God.
The first to come is Moloch, who is covered in blood. He somehow deceived Solomon – an Old Testament king – to build a temple for him.
Next comes Chemos; after the Israelites made it out of Egypt, they started spending a lot of time with non-Hebrew peoples and eventually started worshipping this guy.
With Chemos and Moloch come Baalim and Ashtaroth. Both of these are general words to refer to types of male and female pagan deities found in the region that is now the modern-day middle east, especially Syria, Iraq, Israel, and Jordan.
Astoreth or Astarte, as the Phoenicians called her, also comes with the fallen angels. She was worshipped by Phoenician virgins and also by the Israelites in their promised land!
Thammuz comes next; he was supposedly wounded every year, which caused the river Adonis to become a purplish color because of his blood.
Next comes Dagon, a Philistine sea-god whose upper half is man, the lower fish.
After him comes Rimmnon, a deity worshipped in what is now modern-day Syria.
Next come the bestial and beastly Egyptian gods with animal heads – Isis, Osiris, and Orus.
The last to arrive is Belial; nobody ever built a temple for him, but he can be found everywhere. He loves vice for itself, and is associated with insolent debauchery.
There were a lot of other fallen angels, but it would take forever to name them all, says our narrator. For example, there were also the Olympian gods that the Ancient Greeks worshipped.
A lot of other devils come, and they all look unhappy, though they appear to have some hope left. They are glad to find that Satan is not in total despair.
Satan rekindles their hope with a speech that sounds good but is really a bunch of rubbish (so says the narrator), and he demands that his flag be unfurled.
When all the fallen angels see the flag (it shines like a meteor), the individual squadrons raise their flags, spears, and shields and roar with one loud voice.
The soldiers start marching (silently) to the tune of some hellish pipes, and eventually assemble in front of Satan, waiting for his command.
Satan stands like a tower over his army (the biggest ever assembled); he's still got some of the old fire still left in him, even after falling a really long way.
Satan tries three times to speak to his minions, but he keeps bursting into tears! Satan can cry? Since when?!
Finally he starts speaking, noting that they are brave soldiers and nobody could have foreseen that such an awesome army could ever be defeated.
Don't worry, he tells them, they will rise again, but they can't fight God in the same way. They have to use "fraud or guile" this time.
The rumor-mill says God intends to create another world, and Satan says they should devote their energies to messing with that world.
Satan finishes, and his legions all draw their swords as a sign of approval.
A group of fallen angels led by Mammon – the greedy, money-loving devil – head towards a volcano rich with "metallic ore."
They start digging in it and eventually unearth a bunch of gold.
A second group works to separate the ore from the rock with the help of liquid fire – there's a burning lake nearby just right for the purpose – while a third group pours the ore into a mould.
Eventually, a huge edifice emerges; it looks like a huge temple and has sculptures adorning it, huge pillars, and even a golden roof. It is more magnificent than anything ever seen on earth.
The fallen angels enter the building, now given the name Pandemonium, to have a council. It is swarming with angels, almost like a beehive.
All of a sudden, the fallen angels, which a minute before were bigger than giants, now shrink to the size of little elves or dwarves (this is so that they can all fit inside Pandemonium).
The squadron leaders retain their giant size (they don't shrink) and gather together for the great debate in Hell.