Book 1, for the most part, recounts Satan and his entourage after their fall from Heaven. Book 1 essentially begins "in medias res" or in the middle of the story: after Satan's fall. This is to be considered the first Fall and Adam and Eve's expulsion from Eden is the second Fall. Book 1 begins by invoking "Mans First Disobedience," the loss of Eden, and Christ's restoration of hope for mankind. But the majority of Book 1 deals with Satan's Fall, thus establishing a link between Satan's Fall and Man's Fall. Whereas Satan's Fall is catastrophic, Adam's Fall will eventually be a felix cupla (fortunate fall) because it leads to Christ's sacrifice and redemption for humanity.
In Book 9, Adam and Eve discuss dividing their jobs so they can get more done. Adam says this isn't necessary but then agrees to separate at least for a short time. Satan, in the form of the Serpent, uses this opportunity to tempt Even when she is alone. Adam is first appalled that Eve has eaten the fruit, but doesn't want to live without her and determines that what's done is done: "But past who can recall, or don undoe?" (IX.926). Adam then begins to rationalize that this is not such a bad thing. Adam eats the fruit. He and Eve make love and upon waking, they feel the loss of their innocence and clothe themselves because they feel shame in their nakedness.
Adam and Eve blame each other. She says that he would have been deceived by the Serpent just as she was. Adam admonishes her, saying he warned her about "the lurking Enemie" before she left to go off alone.
Adam realizes his error. He thought he and Eve were perfect and therefore beyond evil seduction. He thought Eve was perfect but she sinned. By mistakenly thinking she was perfect, Adam is also not perfect. Book 9 ends with Adam and Eve continuing to blame each other:
Thus they in mutual accusation spent
The fruitless hours, but neither self-condemning,
And of thir vain contest appeer'd no end.
Adam's role in Book 9 is simply the first man's disobedience. He and Eve are fallen heroes. His Fall is comparable to Satan's. But Satan embraced his fall in rebellion. Adam and Eve dwell on their sin.