The poem opens with Satan, who has just fallen from Heaven and wakes up on burning lake. He realizes he's lost everything and is now stuck in a horrible place. He's so angry at God, he wants to get back at him. He's heard rumors about a new world with some new creatures on it, whom he vows to seduce to his side or destroy.
Satan easily leaps over the wall and into the garden. We don't know what's going to happen at this point, but we do know that he's up to no good. This marks the real beginning of his malicious plans. He just might succeed in ruing Adam and Eve…
Satan easily sneaks into the garden. He doesn't get a chance to do anything except gather some information because God's angels sniff him out and take him away. Adam and Eve are safe…for now.
This is the moment when Adam and Eve finally lose their innocence; immediately after Adam eats the fruit he looks at Eve with "lascivious eyes." In other words, lustfully rather than romantically. They will soon realize that they are naked, cover themselves up with some fig leaves, and be kicked out of Paradise, all because of their eating of the Forbidden Fruit. This is, without a doubt, the most important moment in the poem.
We all know the story of Adam and Even but we can't help hoping that God will be nicer this time. Adam and Eve continue to live as if nothing's wrong, and we continue hoping things won't go according to the Bible story. This isn't the Bible, after all, it's Milton. It's art. Maybe Adam and Eve won't get kicked out of Eden this time.
This is the consequence of Adam and Eve's disobedience, as God has said all along. To make things even more comforting, Adam gets a little history lesson from Michael about all the consequences of he and his wife's actions (mortality, war, murder, every major Biblical event, etc.). All this badness that's about to enter the world is made painfully literal in the bridge that Sin and Death build from Hell to the world. The stakes are really clear now.
The last twenty lines or so of the poem describe Adam and Eve's departure from the happiest place on earth (no, not Disneyland, that hasn't been built yet). They are crying as the gate is closed and a flaming brand is placed over it. This is the end of a pain-free, immortal life for humans. Paradise has been lost.