Lady Macbeth receives a letter from Macbeth, calling her his "dearest partner of greatness," and telling her of the witches' prophecy.
Lady Macbeth says she's worried her husband's not up for killing the current king in order to fulfill the witches' prophecy. Macbeth, she says, is "too full o'th' milk of human kindness" and isn't quite wicked enough to murder Duncan. (Looks like Lady Macbeth isn't going to leave anything to "chance.")
Lady Macbeth says she's going to browbeat her husband into action.
When a messenger enters and announces that King Duncan will stay the night at Inverness as a guest of the Macbeths, Lady Macbeth tells us it'll be King Duncan's last night on earth.
Then Lady Macbeth delivers one of the most interesting and astonishing speeches ever. She calls on spirits to "unsex" her, "make thick [her] blood," and exchange her breast "milk for gall." Translation: Lady Macbeth calls on murderous agents to stop her menstrual flow and change her breast milk for poison (undo all the physical features that make her a reproductive woman). Basically, she suggests that being a woman and a mother could prevent her from committing a violent deed.
When her husband (the guy who's "too full o'th' milk of human kindness") enters the castle, Lady Macbeth tells him that King Duncan's spending the night but he won't be waking up the next morning.