Lady Macbeth reads the letter aloud, in the scene, perhaps for the last of many times. She is Macbeth's "dearest partner of greatness", as he puts it in the letter, and she immediately begins plotting as to how she can achieve what the letter sets out:
Glamis thou art, and Cawdor, and shalt be
What thou art promised.
Yet Lady Macbeth knows that her husband's nature is too full of kindness to "catch the nearest way" by murdering the king. She knows that she will have to persuade him, by pouring her spirits in his ear (an image, to the Elizabethans, of poisoning - as Claudius murders Old Hamlet in "Hamlet"), to do the deed:
Hie thee hither,
That I may pour my spirits in thine ear,
And chastise with the valor of my tongue
All that impedes thee from the golden round
Lady Macbeth, in short, needs to muster up the necessary vehemence and evil to ensure that Macbeth will become what he has been promised - the king. And so she calls on evil spirits to help her
Come, you spirits
That tend on mortal thoughts, unsex me here
And fill me, from the crown to the toe, top-full
Of direst cruelty! Make thick my blood,
Stop up the access and passage to remorse...
Lady M immediately trusts the witches - and sets about seeing that their predictions come true.