Milton best describes what Paradise Lost is in book 1 when he summons the muse. Paradise Lost is Milton's attempt to "justify the ways of God to men." The story opens with Satan and his host in hell after falling from grace, result of a failed rebellion in heaven. After much debating, Satan makes his way through Chaos to Eden, Paradise. There he eventually tempts Eve to eat the forbidden fruit, which leads to man's downfall. The story can't be summarized with those words alone though. Along the way we are told of the primeval war in heaven between God and Lucifer, we learn how and why the world and man were created; the damnation of man, and his redemption. Philip Pullman does a good job of also providing an introduction at the beginning of each book which I found to be helpful while reading the story.