Each book of Paradise Lost is prefaced with an argument, or summary. These arguments were written by Milton and added because early readers had requested some sort of guide to the poem. Several of the books also begin with a prologue. The prologue to Book I states Milton's purpose: to tell about the fall of man and justify God's ways to man.
The epic begins traditionally in medias res. Satan and the other rebellious angels awake to find themselves in Hell on a lake of fire. Satan is lying beside Beelzebub. Satan raises himself from the lake and flies to the shore. He calls for the other angels to do the same, and they assemble by the lake. Satan tells them that all is not lost and tries to inspire his followers. Led by Mammon and Mulciber, the fallen angels build their capital and palace, Pandemonium. The highest ranking of the angels then assemble for a council.
In the council, Satan asks what the demons think should be the next move against God. Moloch argues for open warfare. Belial twists Moloch's arguments, proposing that nothing should be done. Mammon, the materialistic angel, argues that they do the best with what they have. Finally, Beelzebub, Satan's second in command, proposes that the angels try to get at God through his new creation, Man. Beelzebub's proposal, which is really Satan's proposal, is adopted, and Satan volunteers to find the new world and new creatures. He leaves at once, flying to the Gate of Hell. There, he meets his children, Sin and Death. Sin opens the gate for Satan who flies out into Chaos and Night. Sin and Death follow him. Finally, in the distance Satan sees Earth.
God watches Satan approach Earth and predicts his success in corrupting Man. Man has free will. But God omnisciently knows what will happen. God adds that Man can be saved through mercy and grace, but he must also accept the just punishment of death, unless someone takes on death for Man. The Son offers to become a man and suffer death in order to overcome it. The angels rejoice.
In the meantime, Satan, sitting on the edge of the Earth, cannot see the way to Man. Satan disguises himself as a cherub and flies to the sun to talk with the archangel, Uriel. Uriel shows Satan the way to Man.
Looking at Earth, Satan is taken with its beauty but quickly overcomes his sympathy to concentrate on what he must do. He sees Adam and Eve and is entranced with their beauty. As Satan listens to the pair, they talk about God's one commandment that they not eat from the Tree of Knowledge under penalty of death. Satan immediately begins to formulate a plan.
Uriel, on the sun, becomes suspicious of the cherub whose face shows changing emotions and goes to warn Gabriel. Gabriel says that he and his angels will capture any interlopers in the Garden, and late that night Ithuriel and Zephron capture Satan whispering in Eve's ear. The two angels bring Satan before Gabriel, who, with God's help, banishes the tempter from Earth.
When Eve awakes, she tells Adam of her troubling dream. Adam comforts her, reminding her that they are safe if they obey God. God decides to send the angel Raphael to warn Adam and Eve to be wary of Satan. Raphael goes to Earth where he eats with Adam and Eve. After the meal, Raphael tells Adam about the great rebellion in Heaven.
Raphael says that Lucifer (Satan) was jealous of the Son and through sophistic argument got his followers, about one third of the angels, to follow him to the North. There, only one of Satan's followers stood up against him — Abdiel, who returned to God.
Satan attacks God and the Heavenly Host, whose power has been limited by God. Nonetheless, God's forces have little difficulty in defeating the rebels. Michael splits Satan in half, which is humiliating, but not deadly, because Satan, as an angel, cannot die. After the first day of battle, the rebels construct a cannon and begin the second day's battle with some success. God's forces begin to pull up mountains and hurl them at the rebels, burying them and their cannons. God is amused at the presumption of the rebels but does not want the landscape destroyed. He sends the Son forth by himself in a chariot. The rebels are quickly herded into Hell.
Next, Raphael responds to Adam's questions about the creation of the world. The angel explains the day-by-day creation of the world in six days. Then, in an effort to keep the angel engaged in conversation, Adam asks about the motions of the heavenly bodies. Raphael explains that Adam should leave some questions to God's wisdom. Next, Adam describes his own creation, his introduction to Eden, and the creation of Eve. He describes how beautiful Eve is to him and the bliss of wedded love. Raphael gives Adam a final warning about Satan as he leaves.
Having been gone from Eden for eight days, Satan returns, sneaking in through a fountain near the Tree of Knowledge. He takes the form of a serpent to try to trick Man. When Adam and Eve awake, they argue over whether they should work together or alone. Eve finally convinces Adam to let her work by herself. Satan, in serpent's form, approaches Eve and, using clever but fallacious arguments, convinces her to eat the fruit of the Tree of Knowledge. After Eve eats, she reveals what she has done to Adam, who, unable to bear the thought of losing Eve, eats also. Having eaten the fruit, the two are overcome with lust and run to the woods to make love. When they awake, they are filled with shame and guilt. Each blames the other.
In Heaven, the angels are horrified that Man has fallen, but God assures them that He had foreknowledge of all that would happen. He sends the Son to Earth to pronounce judgment on the humans and the serpent. The Son goes to Earth and makes his judgments. He adds though, that through mercy, Adam and Eve and all humans may eventually be able to overcome death. In an act of pity, the Son clothes the two humans.
Sin and Death meanwhile have sensed an opportunity on Earth. They construct a huge causeway from Hell to Earth. On their way across, they meet Satan returning to Hell. They proceed to Earth while Satan enters Hell in disguise. Satan appears on his throne and announces what he has done. Expecting to hear the applause of all the fallen angels, he instead hears only hissing as he and all his followers are turned into snakes. When they eat the fruit of the Tree of Knowledge which appears before them, it turns to bitter ashes.
On Earth, Sin and Death see infinite opportunities. God, looking down on the two, says eventually they will be cast into Hell and sealed up. Adam and Eve lament, but Eve submissively asks Adam's forgiveness. He relents, his love overcoming his bitterness. She suggests suicide as a way to avoid the terrible curse on the world, but Adam says they must obey God.
God sends the angel, Michael, to take Adam and Eve out of Eden. Before doing so, Michael takes Adam to a hill and gives the human a vision of biblical history, ending with the birth of Jesus who will be the savior of Man. Adam rejoices. Adam and Eve together are led out of Eden. Behind them a flaming sword guards the entrance; ahead, they face a new life in a new world.