The following is excerpted from the eNotes charcter analysis portion of "King Lear" which explains the inter-connection between the sub and main plot.
Gloucheter's situation parallels the situation of Lear. He mistakenly believes Edmund when the latter pretends to read a letter that is falsely said to be written by Edgar. In that letter, Edgar supposedly tells Edmund of his impatience to inherit Gloucester's estate. Gloucester, like Lear, responds emotionally, immediately denouncing his legitimate son (Edgar) and trusting in the son who really intends to do him wrong (Edmund). And like Lear, Gloucester is to be punished for his lack of insight or moral vision. That punishment comes in the form of a brutal incident wherein his eyes are ruthlessly plucked out by Cornwall. The physical blinding of Gloucester is symbolic of both his own and Lear's blindness to the truth about their children.
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we were asked this for our english class and i dont know if this is write so im welcome to comments about it.
i think if you look at the loss of land and gulibility (is that a word) of gloucester you can understand lears plot. Gloucester believes Edmund so easily when he denounces Edgar regarding selfish issues and lear does too when Cordelia refuses to speak in the love test. The loss of land i think is important also as the childrens lust for the land is affected by how much is there. If Edmund can decieve his father for his section of land then Goneril and Regans actions can be explained because there is a lot more land at stake. I think that this also explains the sibling rivalry too.
Because of this difference in land and loss of land between gloucester and lear explains the difference in severity of their punishments or end results, with gloucester losing his sight and lear his sanity. i think the sub plot does compliment by suplying a sort of comparison for the reader to go by. Gloucester lost his title and sight but Lear was king and the sub plot highlights Lears drop in status and sanity. Ithink its basically a comparison.
(just a note: we havent read after act 3 so dont judge too harshly i still dont know how it ends. haha)