King Lear opens with a conversation between the earls of Kent and Gloucester, in which the audience learns that Gloucester has two sons: Edgar, who is his legitimate heir, and Edmund, his younger illegitimate son. This information will provide the secondary or subplot. Next, King Lear enters to state that he intends to remove himself from life's duties and concerns. Pointing at a map, Lear tells those in attendance that he has divided his kingdom into three shares, to be parceled out to his three daughters, as determined by their protestations of love. The two elder daughters, Goneril and Regan, exaggerate their love by telling their father that their affection for him exceeds all reasonable expectations. The youngest daughter, Cordelia, tells Lear that she loves him, but only as a daughter should love a father. Lear, angry and disappointed at what he deems a lack of devotion on Cordelia's part, divides his kingdom equally between Goneril and Regan, and banishes Cordelia. Later, France agrees to marry the now dowerless and banished Cordelia. When Kent attempts to defend Cordelia, Lear banishes him as well. Meanwhile, Goneril and Regan decide that if Lear becomes too much of a nuisance, they will have to decide what disciplinary actions to take.
In the developing subplot, Edmund complains of his unhappiness at being an illegitimate — and thus, disinherited — son. As part of his plot to claim what is not his, Edmund gives a false letter to his father, Gloucester, declaring that Edgar is proposing that they kill their father and split the wealth between them. The cunning Edmund easily convinces his father that Edgar cannot be trusted.
Within a short time, Lear moves to Goneril's palace. Goneril tells Lear that he needs a smaller troop, more decorous in behavior and better suited to the king's rank and age. The king is very angry and says he will pack up his people and move to Regan's palace. Lear's anger continues to build, and he calls upon nature to curse Goneril's womb. In response, Goneril turns out 50 of Lear's retinue.
As the subplot develops, Edmund wounds himself slightly, pretending that Edgar has attacked him. Certain that Edgar will also try to kill him, Gloucester promises to find the means to make Edmund his heir. After his escape into the woods, Edgar decides that he will disguise himself as a Bedlam beggar, who will be known as Poor Tom. Meanwhile, Cornwall orders an impassioned Kent placed in the stocks. Lear arrives and quickly realizes that Regan has joined Goneril in seeking to reduce Lear's authority. Lear reminds his daughters that he gave them all that they now enjoy, but they are unmoved. An angry Lear calls for his horse, and rides into the storm with his Fool for protection. Exposed to the storm, the Fool attempts to reason with his king, but Lear will have no part of submission, especially before his daughters. Soon the king and Fool are joined by Edgar disguised as Poor Tom.
Gloucester tells Edmund of the plot to save the king, unaware that he is divulging the plans to a traitor. Edmund immediately resolves to tell Cornwall of the plan. Edmund soon receives his reward: Gloucester's title and lands. The captured Gloucester is tortured by Regan, who fiendishly plucks at his beard, and Cornwall, who gouges out Gloucester's eyes, but not before one of Cornwall's servants draws a sword and stabs Cornwall, who soon dies of his wounds.
Later, Edgar is both shocked and dismayed when a blinded Gloucester is led in by one of his tenants. The disguised Edgar agrees to take Gloucester to the cliff he seeks, where he dupes Gloucester into thinking that he is at the edge of a precipice. After Gloucester jumps and loses consciousness, Edgar easily convinces his father that he has somehow survived a fall from the cliffs. Oswald arrives and attempts to kill Gloucester but is, instead, slain by Edgar. As he lays dying, Oswald gives Edgar a letter from Goneril instructing Edmund to murder Albany so that she will be free to wed Edmund.
Goneril and Edmund soon learn that Albany is a changed man, one who is pleased to learn of the proposed invasion by France and displeased when he learns that Gloucester has been replaced by his younger son, Edmund. Meanwhile, Cordelia learns of her father's deteriorated mental condition and returns to England with an army to defend her father. Within a short time, Cordelia and her father reunite.
In spite of Albany's intent to save Lear and Cordelia's lives, Edmund resolves that they will die. Edmund orders that Lear and Cordelia be imprisoned. Albany, Goneril, and Regan join Edmund, and a confrontation erupts between all four characters. Edmund's treachery is revealed, and he is wounded in a fight with Edgar, whom Edmund does not recognize as his brother. Soon, Regan dies, poisoned by Goneril, who then kills herself. Since he is now dying, Edmund admits that the charges against him are truthful, and he seeks to know the identity of his killer. Edgar confesses his lineage as brother and shares the news that their father, Gloucester, has died.
Edmund, who says he wants some good to come from so much death, reveals his and Goneril's plan to have both Lear and Cordelia murdered and to have Cordelia's death appear a suicide. Efforts to rescind these orders are too late, and soon Lear enters with a dead Cordelia in his arms. Unable to accept Cordelia's death, the king also dies, his body covering that of his youngest daughter. Albany informs Kent and Edgar that they must now rule the kingdom together, but Kent replies that he will soon leave the world to join his master. Edgar is left to speak of the sad weight of these events, which everyone must now endure.