Lady Macbeth asks a servant if Banquo is already gone. When she realizes he has, she 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 he will. And he tells his wife she should say a lot of really nice things about Banquo, flatter him, and maybe even flirt with him a little. That will help hide their guilt.
Lady Macbeth tells him he has to stop talking about what they've done.
But Macbeth says that as long as Banquo and Fleance are alive, he's going to be paranoid. He can't stop these dark thoughts and his fear of being found out, and his worries about Banquo's son getting his crown.
Lady Macbeth says they won't live forever, which leads Macbeth to say, "Hmm. That's true. In fact..."
In fact, what? Lady Macbeth wants to know what her husband is plotting.
Macbeth dodges her 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. Gee. 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.