Full circle
I'm Edmund. I'm Gloucester's illegitimate son, and he never lets me forget it. So I decide to frame my half-brother so my father will like me instead of him. I'm ruthless, cunning, and a little crazy. And you know what I think?
Thou hast spoken right, 'tis true;
The wheel is come full circle: I am here. (5.3.171-172)
Who Said It and Where
A guy named Gloucester is in the habit of running around town calling his illegitimate son, Edmund, a "bastard" and cracking dirty jokes about Edmund's unmarried mom. So we can't say we're surprised when Edmund begins to scheme against his dad and his half-brother Edgar, who is Gloucester's so-called legitimate son.
Meanwhile, King Lear's daughters Regan and Goneril are fighting him for the crown. Edmund decides to join up with them and throw down with the old man. When they all meet up for a victory conference, they praise Edmund for his bravery in battle—he's clearly the one responsible for their triumph.
With the real battle over, however, the sisters' battle over the evil yet oh-so scrumptious Edmund can only ramp up. Regan claims Edmund as her future husband, and she and Goneril scuffle about it—in veiled terms—since Goneril's husband is standing right there.
Albany, annoyed that Edmund has risen so quickly in the world, reminds him that he's only some illegitimate son of a lord. Then Albany plays his trump card: he arrests both Edmund and Goneril for treason. Zing. He reveals he knows they've been plotting against his life so they can get married. Albany orders that the trumpet sound three times—if nobody comes to challenge Edmund, then Albany will just have to challenge Edmund to a duel himself.
Edgar rushes in dramatically at the third trumpet call, and, still in disguise, challenges Edmund to a duel. In the duel, Edgar stabs Edmund in the guts. Goneril freaks out because Edmund is hurt. When Albany tries to confront her about her plot to murder him, she runs offstage.
Edmund, mortally wounded, admits that he's guilty of the charges. He wants to know the identity of the man who killed him. Edgar finally reveals himself ("Edmund, I am your brother") and tells his story. Edmund is moved by his brother's speech, even if he did frame the guy and plot against him this whole time.