"When shall we three meet again, in thunder, lightning or in rain?" is the opening line of the famous play "Macbeth" written by William Shakespeare. The line is spoken by the three witches, also known as the "weird sisters", who are a crucial part of the play's plot. They prophesy Macbeth's rise to power and eventual downfall, setting the course of events into motion. This opening line sets the ominous and foreboding tone of the play, hinting at the dark and supernatural elements that are to follow.