ACT I opens on a desolate heath where three witches meet and chant “Fair is foul, and foul is fair.” In a nearby Scottish camp, King Duncan, his sons Malcolm and Donalbain, and noblemen learn from a wounded captain that Macbeth and Banquo have won a great victory over the rebel Macdonwald and the Norwegian forces. The Thane of Cawdor has been executed for treason, and Duncan orders the title passed to Macbeth.

Later on the same heath, the witches summon Macbeth and Banquo. They hail Macbeth as Thane of Glamis (his current title), Thane of Cawdor (newly granted), and future king. Banquo receives a cryptic blessing that his descendants will be kings. Macbeth, stunned, demands more information; the witches vanish, leaving him uneasy.

Ross and Angus arrive to announce to Macbeth that he has indeed been named Thane of Cawdor, confirming the witches’ prophecy. Macbeth reflects on the “two truths” and begins to suspect that the witches’ words may be a trap, while Banquo worries that dark forces may be manipulating them.

In King Duncan’s palace at Forres, Duncan declares his eldest son Malcolm the Prince of Cumberland, signalling the line of succession and presenting a new obstacle to Macbeth’s ambition. Macbeth outwardly thanks the king but internally vows that “the Prince of Cumberland” is an “obstacle” he must overcome.

In Inverness, Lady Macbeth reads a letter from Macbeth describing the witches’ prophecies. She fears her husband’s nature is too kind and resolves to “unsex” herself, calling on spirits to fill her with cruelty. A messenger tells her that King Duncan will stay the night at their castle. Lady Macbeth urges Macbeth to appear hospitable but secretly encourages him to murder Duncan. Macbeth arrives, announces Duncan’s impending visit, and Lady Macbeth begins to plot the king’s murder, urging him to hide his intentions.

Duncan arrives at Macbeth’s castle with his retinue and praises the hospitality. Lady Macbeth greets him with exaggerated courtesy. Shortly after, Macbeth soliloquizes about the moral and practical consequences of murdering Duncan, recognizing the act’s treachery and his own lack of resolve. Lady Macbeth confronts his hesitation, belittles his courage, and outlines a detailed plan: they will intoxicate Duncan’s servants, frame them for the murder, and hide the daggers. Macbeth, spurred by Lady Macbeth’s resolve, declares he is settled on the deed and that “false face must hide what the false heart doth know.” The couple exits, prepared to carry out the regicide.