Your answer may include the following points:
The plot in act II is crucial because it contains Duncan’s murder. Act II has two scenes, the first set in Macbeth’s castle in Inverness.
In the previous act, Banquo praised Inverness for its peaceful atmosphere, but he now talks of the night being darker than ever. Banquo is worried that the witches' prophecies will lead him to ambitious thoughts. He is inside Inverness, where the plan to murder Duncan has already been hatched.
Though the next scene is also in Inverness, the murder of King Duncan is not shown onstage. When Lady Macbeth hears moans from Duncan’s room, so does the audience. This adds to the suspense of the moment.
Later, after Macbeth emerges with bloody hands, Lady Macbeth goes inside to place the bloody daggers near the chamberlains. When she returns to the stage, her hands are covered in blood. The erstwhile peaceful atmosphere of Inverness has given way to a bloody and sinister night. The porter’s thoughts on what it would have been if he had been "hell’s gatekeeper" suggest that after the murder of Duncan, Inverness has turned into hell.
Lennox comes from outside, but the atmosphere he describes isn’t very different. He witnessed a stormy night with the wind bearing cries of death.
Offstage, Macduff discovers the murder.
The second scene, which is set outside the castle, has an unnaturally disturbed atmosphere in sharp contrast to the peace of the earlier scene when Duncan arrived. This contrast suggests the breakdown of the natural order because of Duncan's murder. The change might symbolize how the evil inside Inverness is about to spread to the rest of Scotland.
