Duncan, Malcolm, Donalbain, Banquo, Lennox, Macduff, Ross, and Angus arrive at Inverness. Duncan comments on the sweetness of the air. Banquo notes that martlets, a species of bird that usually nests in churches, have nested in the castle.
Ironic that Duncan thinks the castle where he'll be murdered is beautiful. Also shows what beauty Macbeth loses when he gives in to his ambition.
Lady Macbeth warmly greets the King and the thanes, though Macbeth is nowhere to be seen.
At this point, the planned murder weighs more on Macbeth than on Lady Macbeth.