Edmund, who has succeeded in capturing Lear and Cordelia, orders his guards to take them away until he figures out what he's going to do with them.
All defiance, Cordelia demands to be taken before her wretched sisters. "No, no, no, no, let's away to prison," Lear tells her. In a moving (albeit delusional and slightly inappropriate) speech, Lear says, "We two alone will sing like birds i'the'cage... we'll live / And pray, and sing, and tell old tales, and laugh / At gilded butterflies."
Lear tells Cordelia that he is no longer interested in politics and court manipulation. In prison, he tells his daughter, the two of them will watch and laugh as different political factions engage in an endless struggle for dominance. Power doesn't matter to him anymore, he says; what he cares about is being with his beloved daughter.
Edmund orders that the prisoners be taken away. He then writes his captain an order on a piece of paper and tells him that he will be promoted if he executes Lear and Cordelia.
Albany, Regan, and Goneril enter for a victory conference. They all praise Edmund for his bravery in battle—he's clearly the one responsible for their triumph.
Albany asks Edmund to hand over Lear and Cordelia, but Edmund distracts him from the issue, saying that Lear looked so pathetic that he had to send him away because the British troops might have felt sorry for him and rebelled.
Edmund says also that now's not the most appropriate time to pass down judgment on Lear and Cordelia, seeing as how so many people are bleeding from battle wounds and counting up their dead friends.
Albany tells Edmund that they're not equals in this war—Edgar is his subordinate, but Regan disagrees. This exchange sets off a tiff between the sisters over the evil yet oh-so scrumptious Edmund. Regan, who mentions that she isn't feeling so great, basically claims Edmund as her future husband, and she and Goneril scuffle about it—in veiled terms, since Goneril's husband is standing right there.
When Goneril gets upset by the idea that Regan plans to "enjoy" (sleep with) Edmund, Albany tells her that it's not her place to object. She's not in charge, and—ahem—she's married, so she shouldn't be getting competitive over this shmuck.
Edmund tells Albany to butt out, and Albany reminds him that he's only some illegitimate son of a lord.
Regan tells Edmund to fight Albany on her behalf, but before Edmund can respond, Albany plays his trump card: he arrests both Edmund and Goneril for treason. Ah-ha! 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.
Meanwhile, Regan's still belly-aching about how she's not feeling so hot. Goneril snickers and reveals to the audience that she's poisoned her sister.
Edgar rushes in dramatically at the third trumpet call, and, still in disguise, challenges Edmund to a duel.
In the midst of all this drama, Regan has to be escorted back to her tent. Goneril watches happily as her sister—her evil plan to poison her sister and secure marriage to Edmund seems to be working.
In the duel, Edgar stabs Edmund in the guts.
Albany tells Edgar not to kill Edmund—if he dies, Albany won't be able to throw him in prison. Goneril is freaking out because Edmund is hurt, and 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. He explains that roughly half an hour ago, when he finally told Gloucester he was his son, Gloucester had a heart attack from a mixture of shock and joy. (Gosh, the body count just keeps rising.)
"This speech of yours hath moved me, / and shall perchance do good" Edmund says.
Then a man runs onstage screaming and holding a bloody knife. Someone has died.
The knife-wielding man reveals that Goneril confessed to poisoning her sister and then stabbed herself. Edmund admits that he was promised to both sisters. Now that all of them are dead or dying, Edmund says, "All three / Now marry in an instant." In other words, the two sisters are dead and Edmund's not far behind.
The soldiers bring out the dead bodies of Regan and Goneril, just so we can really visualize the whole thing.
Kent walks in and asks everybody where Lear and Cordelia are. Uh-oh, says Albany. We totally forgot about Lear and Cordelia!
Looking at the corpses of Regan and Goneril, Edmund says proudly, "Yet Edmund was beloved."
But then Edmund (who's not quite dead yet) decides to do something good for a change. He suddenly confesses that he ordered his captain to have Lear and Cordelia killed. If Albany sends someone lickety-split to stop the Captain, maybe they can save Cordelia from being hanged.
Edgar dashes off to intervene, and everyone else onstage waits tensely to find out if he is too late. "The gods defend her," Albany prays.
The answer to Albany's prayer is the sound of Lear howling. The old King staggers onstage with his daughter in his arms. Cordelia is dead.
Lear keeps asking for some way to check if Cordelia is still breathing—a mirror to look for the mist of her breath, or a feather that might move when she exhales.
But really, Lear knows that it's too late. "A plague upon you murderers, traitors all," he curses. "I might have saved her; now she's gone for ever."
Kent tries to comfort Lear, and reveals himself as Lear's guardian in disguise. But Lear brushes him off—he is too preoccupied with the death of his daughter to understand what Kent is trying to say.
After sacrificing everything to help the King, Kent doesn't even get the satisfaction of Lear recognizing his devotion.
Meanwhile, a Gentleman enters and announces that Edmund is dead. Whatever, says Albany, who tries to address the political situation. He tells Lear that he can be king again, but no one is listening to him.
Lear still holds his daughter's corpse in his arms. "Why should a dog, a horse, a rat have life / And thou no breath at all?" he asks. "O thou'lt come no more, / Never, never, never, never, never."
What happens next in King Lear is a bit tricky because there are two different versions of the play (three if you count the "conflated" text, which shmooshes the two versions together into one big, long play). In the First Folio edition (the collected works of Shakespeare published in 1623), Lear dies thinking that Cordelia is dead and Albany gets to speak the final lines of the play.
In the First Quarto edition of Lear (printed in 1608), Edgar (not Albany) delivers the final lines and Lear dies believing that Cordelia is alive. Here's what goes down in this version:
As Lear attends to Cordelia's body, he thinks she's still breathing—"Do you see this? Look on her: look, her lips / Look there, look there!" he says, and dies.
With Lear dead, the kingdom needs a ruler. Albany suggests that Kent and Edgar share the throne and help England to heal.
Kent refuses, saying ambiguously that he's got to follow his master, hinting that he'll go with Lear on his journey into death.
Then Edgar says "The weight of this sad time we must obey / Speak what we feel, not what we ought to say. / The oldest hath borne most. We that are young / Shall never see so much, nor live so long." In other words, Edgar says we're all going to get old and die. In the meantime, we should all be honest and say what's in our hearts instead of running around lying all the time.