Lady Macbeth asks a servant if Banquo is already gone, and finding he has left, asks the servant to get Macbeth for a little chat.
Macbeth comes along, and Lady Macbeth tells him to look more chipper and not dwell on dark thoughts, as "what's done is done."
Macbeth points out they've merely scorched the snake, not killed it. Macbeth compares dead Duncan's death as a state preferable to his; at least Duncan doesn't have to worry about loose ends.
All right, Debbie Downer, says Lady Macbeth; just chill out there.
Macbeth says she should say a lot of really nice things about Banquo, who will be otherwise engaged and not attending the dinner party. ("Otherwise engaged" = dead.) Um, isn't that a bad idea? Nah, says Macbeth.
As Banquo and Fleance live, his mind is full of scorpions. Lady Macbeth states that everybody dies, which may be a warning to Macbeth to cool it, or may be a self-reassurance that everyone has to go sometime, so her husband might as well murder their friend and his kid.
But really, Macbeth says, he's about to do something bad. In one of her less astute moments, Lady Macbeth asks what that naughty thing might possibly be.
Macbeth dodges the question, saying it's better for her to "be innocent" and not know his plans until they're accomplished and she can applaud him for it. Hmm. It seems like Lady Macbeth no longer gets any say in her husband's affairs.
Macbeth appeals to nature to let night's black agents do their thing, and then he exits with Lady Macbeth.