The subplot of King Lear focuses on the Earl of Gloucester’s loss of power as he misjudges his children’s intentions with regard to their inheritance. Gloucester’s illegitimate son Edmund convinces him that his legitimate son Edgar is plotting against him. After transferring power to Edmund, Gloucester realizes his mistake and eventually pays the physical price of losing his eyes.

This subplot both mirrors and provides depth to the main plot surrounding the aging King Lear and his children’s desire for his power. Lear is swayed by Goneril and Regan to disown Cordelia and leave her out of inheriting the kingdom. Cordelia is the only daughter who is honest and caring for her father, while Goneril and Regan simply want power, just like Edmund. Goneril and Regan begin a war which ultimately leads to King Lear’s death.

The purpose of including the subplot helps show that this experience is a human experience and not one solely affecting King Lear. It shows that no matter the amount of power one has, there are people who are willing to act nefariously to usurp that power. It also shows how the quest for power can easily corrupt an individual, as Edmund, Goneril, and Regan all become increasingly corrupt as they get closer to gaining the power they seek.

The subplot also shows that wisdom can come from painful experiences. When both Gloucester and Lear understand the treason levied against them, they are able to gain a new understanding and appreciation for the mistakes they made in not trusting their loyal children.

The Gloucester subplot mirrors the main plot of the drama. In the main plot, Lear is betrayed by his two elder daughters because he believes their flattering words and gives them his power. He thinks they will continue to honor and care for him. Likewise, Gloucester is duped by his illegitimate son Edmund into turning on his legitimate son Edmund—and ends up similarly betrayed by one he believed he could trust.

Both men thought they were wise in the ways of the world, and both, humbled, find themselves cast out into harsh nature to face themselves and the world without the trappings of power and position.

The subplot fulfills several functions. First, it shows that the betrayal Lear experiences is not unique: the young will betray the old, and those with power must be careful not to be too trusting. The desire for power is a huge temptation. Edmund, Goneril, and Regan all believe they deserve the power they have grabbed and that they are justified in using it immorally to achieve their ends. All three become more corrupt as they gain more power, until, for example, Albany rejects Goneril as nothing more than a fiend or monster.

In contrast, suffering and loss of power brings characters like Lear and Gloucester to greater wisdom and insight. Because these two men are undergoing the same experience, they are able to talk and commiserate with each other and with Edgar. In the end, these two once great men accept the reality of their common humanity; like everyone else they are, as Lear puts it, looking at Edgar:

no more but such a poor, bare, forked animal as thou art.

Having more than one person going through the same experience gives Shakespeare scope to explore it more fully and with more drama.

Gloucester acts as a foil to Lear. This means that he provides a mirror to the main character, whose traits are then highlighted for dramatic purposes. In structural terms, there are great similarities between the two men and their respective fates. Both come to grief through a naive trust in their children; both suffer appalling pain: mentally in the case of Lear, physically and mentally in the case of Gloucester; both realize, all too late, which family member was the one who really loved and cared for them.

Yet there are important differences between plot and subplot. Gloucester's suffering is more recognizably human than the mental anguish endured by Lear. Lear remains a larger-than-life character throughout the play; he may have renounced his kingdom, but he still acts and feels like a king. His lapse into insanity stems largely from his inability to come to terms with his humiliating status as a king without a throne.

Gloucester is more human and less exciting as a character, perhaps, but none the worse for that. And unlike Lear, he gives—not out of an insecure need to be loved, but out of a genuine sense of humanity. Both men ultimately fall, but for different reasons: Lear cannot live in a world in which he has no place because he is too God-like; Gloucester, a mere mortal, cannot live there either, but because he's all too human. The Gloucester subplot reminds us that what happens to Lear can happen to any of us, albeit for different reasons. 

In King Lear, the Earl of Gloucester’s storyline resembles Lear’s. In both cases, a powerful older man thinks he knows his children better than he does. They turn against their loyal children and are duped by their duplicitous children. Lear disowns the honest Cordelia and gives his kingdom to the flattering Goneril and Regan. Gloucester’s illegitimate son Edmund tricks him into believing his legitimate son Edgar is plotting against him. Goneril and Regan strip Lear of power, shut him out in the cold, and defend their actions in a war that leads to Lear’s death. Due to Edmund’s scheming, Gloucester loses his eyes, and his heart eventually gives out.

There is redemption in both of their stories, however. Lear reunites with Cordelia before they die, realizing that he is “a very foolish fond old man.” Edgar also meets Gloucester, cures him of his desire to kill himself, and eventually reveals himself to him. This revelation gives Gloucester joy but also kills him: his heart, “'Twixt two extremes of passion, joy and grief, / Burst smilingly.” One of the lessons learned in both plots is that a man, especially a powerful one, can be too comfortable in his ignorance. Gloucester is less ornery than Lear, but he is careless about Edmund’s mental state. Both fathers are humbled to discover how deceiving appearances can be, even in their own families.