What part of the plan has Macbeth failed to accomplish?
Each of the Macbeths has a role to play in Duncan's murder. Lady Macbeth is going to give the guards drugged wine, and when they're knocked out she's going to lift their daggers and place them where Macbeth can find them. Macbeth will then slip into the chamber, kill Duncan with the grooms’ daggers, and then plant the daggers on the unconscious grooms to make them look guilty.
All goes according to plan until Macbeth reunites with his wife. She realizes that rather than leaving the daggers planted on the grooms, he's brought them back with him and is holding them in his hand. The daggers in his bloody hands would incriminate him immediately in the murder, should anybody see him, but he’s too horror-struck to do anything about it, and is too frightened to return them. Lady Macbeth stays levelheaded, though, demanding, “Give me the daggers". She then takes off to plant them on the grooms, then returns to help Macbeth clean up the blood. She’s quite steely and calm in this scene. Over the course of the play we will see her steady exterior unravel as the blood she cleans in this scene begins to haunt her imagination.