A porter goes to the answer the door, joking to himself that he is the doorkeeper at the mouth of hell, and mocking whoever might be knocking to get into hell. At the door are Macduff and Lennox. Macduff good-naturedly asks what took so long. The porter blames drunkenness, and makes a series of jokes about alcohol and its effects on men.
The Porter provides a moment of ironic comedy. He imagines he's guarding hell, but with the murder of Duncan he really is guarding a hellish place.
Macbeth enters, pretending to have just woken up. Macduff asks if the King has woken yet: Duncan had asked to see Macduff early that morning. Macbeth points out where Duncan is sleeping, and Macduff goes off to wake him.
Introduction of Macduff, and contrast between Macbeth's lying and treachery with Macduff's openness and loyalty.
As they wait for Macduff to return, Lennox describes the terrible storm that raged the previous night and sounded like "strange screams of death" (2.3.52).
The unnatural act of killing Duncan has caused havoc in nature.
Macduff cries out in horror and runs onstage. Macbeth and Lennox ask what happened, then run to Duncan's chamber. Banquo, Malcolm, and Donalbain wake. Lady Macbeth enters, pretending not to know what happened, and expressing horror when Macduff tells her of the murder. Macbeth returns, and wishes he had died rather than have to see such a thing. Malcolm and Donalbain enter and ask what's happened. Lennox tells them that Duncan was murdered by his drunken attendants.
Everyone is being "natural" and honest in their grief except Macbeth and Lady Macbeth. They are the snakes hiding behind the "innocent flower." Everything they do now must protect their secret. This secrecy becomes their defining trait, warping them.
Macbeth wishes aloud that he hadn't killed the attendants. When Macduff asks why Macbeth did kill the attendants, Macbeth says he was so furious that they had murdered the Duncan that he couldn't control himself. Lady Macbeth faints.
Macbeth killed the attendants to keep them quiet. Does Macduff suspect already? Lady Macbeth faints to head off further questioning.
The thanes agree to meet in the hall to discuss what's happened. Malcolm and Donalbain, though, remain behind. They realize that one of the thanes is probably the murderer and fear that they'll be the next targets. They decide to flee: Malcolm to England and Donalbain to Ireland.
Malcolm and Donalbain realize any one of the thanes could be faking his grief. The unnatural hides itself by looking natural.