The porter delays the opening of the gate after hearing the knocks. (which was really entertaining to the audience). It turned out to be Macduff. Macduff talks to Macbeth about all the terrible things that occurred in nature that night. Macduff finds out about the death of the King. Macbeth and Lady Macbeth do hide the involvement of them in that crime.

Lady Macbeth:

Woe, alas! What, in our house

…

Macbeth:

Had I but died an hour before this chance, I had lived a blessèd time, for from this instant there’s nothing serious in mortality. All is but toys. Renown and grace is dead. The wine of life is drawn, and the mere Lees is left this vault to brag of.

Duncan’s sons are travelling to England and Ireland, because they think that they are next in the list of killing.

Shakespeare does this brilliant technique of entertaining, which is to switch between comedy and drama so that there is not that killing involved and so that the act of killing will be interesting.