Paradise Lost opens with Satan on the surface of a boiling lake of lava in Hell (ouch!); he has just fallen from Heaven, and wakes up to find himself in a seriously horrible place. He finds his first lieutenant (his right-hand man), and together they get off the lava lake and go to a nearby plain, where they rally the fallen angels. They have a meeting and decide to destroy Adam and Eve (God's children and precious science experiment) in order to spite God. Satan volunteers for the job and leaves Hell to go look for Adam and Eve.
The scene then shifts to Heaven (Book 3), where God talks about how he can see what Satan is planning. He knows everything all the time. He has a conversation with His Son, says he knows that Satan will tempt mankind and that Adam and Eve will eat the fruit of the Forbidden Tree. He needs to know if anyone will intervene on man's behalf. The Son volunteers, which makes God and all the angels in Heaven very happy.
The scene shifts again, this time to Eden. Satan has reached the Garden, and we see Eden and Adam and Eve for the first time through his eyes. We watch Adam and Eve hang out together for a while, before going into their hut to go to bed and make love. Meanwhile, God has sent out a search party to get Satan out of the Garden, which is easy as pie. The next day, God sends the angel Raphael to talk to Adam and Eve about Satan and whatever else they might want to know. About a week after Adam's chat with Raphael, Satan returns to the Garden, disguises himself as a serpent (snake), and convinces Eve to eat the Forbidden Fruit. She in turn convinces Adam to have a taste. After that, they have steamy, lustful sex for the first time.
As a result of Adam and Eve's sin (eating the Forbidden Fruit), the gates of Hell are now wide open for Sin and Death (who are actual characters in this poem) to build a bridge from Hell to earth. Satan returns to Hell triumphant, but he and his angels are eventually turned into serpents as punishment for Satan's evil deed.
As for Adam and Eve's punishment, God makes them leave the Garden of Eden. He also introduces death, labor pains, and a bunch of other not-so-fun stuff into the world. Before they leave Paradise, however, God sends the angel Michael down to give Adam a vision of the future. After his history lesson, Adam and Eve leave the Garden of Eden in what is one of the saddest moments in English literature.