King Lear is set in Britain in an unspecified period of the Middle Ages. Everything in King Lear unfolds from two overarching plot elements. First, Lear is old. He wants to step down from the throne and divide his kingdom among his three daughters. Second, the Earl of Gloucester has an illegitimate son, Edmund, who is unhappy with his bastard status and decides to displace his older brother and the legitimate heir, Edgar.
Lear holds a public ceremony during which he asks his three daughters to declare their love for him. The older two daughters, Goneril and Regan, make great declarations of love. Each receives a third of Lear's kingdom. Lear's youngest daughter, Cordelia, objects to her sisters' false flattery and won't take part in the ceremony. Outraged, Lear strips her of her dowry and divides the remaining third of the kingdom between Goneril and Regan, oblivious to the insincerity of his elder daughters. The King of France is one of Cordelia's suitors, and he accepts her as his wife without a dowry or her father's approval. Banished by her father, Cordelia goes to France and becomes queen. When the Earl of Kent objects to Lear's treatment of Cordelia, Lear banishes him as well.
Meanwhile, Edmund convinces his father that Edgar is plotting treason, and Gloucester exiles Edgar under pain of death. Edgar disguises himself as a crazy beggar, calling himself Poor Tom, so he can stay in the kingdom.
Kent remains loyal to Lear, and he disguises himself so he can stay with the king and serve him. Lear's plan is to divide his time between his elder daughters' estates, accompanied by 100 knights. In reality, however, neither daughter wants him in her home, and both take actions to cast him out. First, Goneril and Regan object to his knights' conduct and the fact that he has so many serving him. When Kent disciplines Oswald, Goneril's steward, she and her husband, the Duke of Albany, betray Lear's authority by putting Kent in the stocks. Finally, when the sisters suggest their father put aside most (or all) of his 100 knights, Lear rages off into the night.
Lear wanders in a storm accompanied by the Fool. Kent (in disguise) soon finds the king and helps him. Between the storm and his daughters' treachery, Lear's mind is in turmoil, and he begins to go mad. While the king and the two other men are taking shelter from the storm in a hut, Edgar (disguised as Poor Tom) joins them.
Kent secretly updates Cordelia on her father's situation. When Gloucester tells Edmund that French forces have landed in England to help restore Lear to the throne, Edmund sees a way of gaining his inheritance more quickly. He betrays his father by telling Regan, Goneril, and their husbands that Gloucester is working with the French. When the Duke of Cornwall (Regan's husband) learns that French forces have landed, he and Regan punish Gloucester by gouging out his eyes. One of Cornwall's servants is so upset by this that he tries to stop them. He wounds Cornwall fatally but is also killed.
Regan and Cornwall (who dies soon after) turn out Gloucester, now blind, to wander through the countryside. One of his peasants helps him at first, and then Edgar, disguised as Poor Tom, takes over that task. Gloucester asks Edgar to guide him to the high cliffs of Dover so he can jump off and commit suicide. Edgar says he will, but he leads his father instead to a low ledge and tricks him into thinking he has survived a great fall.
During an update on the battle, a split between Goneril and her husband, the Duke of Albany, is revealed; they are quarreling, and she accuses him of cowardice. While they are fighting, a messenger arrives and reports that the Duke of Cornwall is dead. When Albany asks why and learns Cornwall was killed for blinding Gloucester, he praises Gloucester's loyalty. This indicates that Albany's loyalties lie with Lear, finalizing the split with his wife.
Cordelia returns from France and joins forces with Kent. She takes care of her father as a battle looms between French forces supporting Lear and rebel forces led by Edmund and backed by Regan and Goneril. During this period, Lear largely regains his sanity.
As the battle nears, the rebels are quarreling. At some point, Edmund started having affairs with both Regan and Goneril. Regan is so obsessed with having him that she doesn't care if the rebels win or lose the battle.
Edmund's forces capture Lear and Cordelia. Edmund sends them to prison and then sends a messenger with orders to kill them. Albany charges Edmund with betraying the king and says he'll prove Edmund's treason through one-on-one combat. An anonymous knight appears and volunteers to fight Edmund. The knight wins the contest, wounding Edmund mortally. The knight then reveals he is Edgar, Edmund's brother. Their father, Gloucester, dies after hearing this. Edmund dies a few moments later.
A servant enters to announce that Goneril has poisoned Regan and stabbed herself. Lear enters, carrying Cordelia. She's dead, as is Lear's Fool, and Lear has killed the man who executed Cordelia. Lear dies of grief. Albany tells Kent and Edgar they will rule the kingdom together until order has been restored.
