Edmund stands alone on stage, criticizing the injustice of the laws and customs that deprive him of all legal rights just because he was born out of wedlock. Therefore, Edmund says, rather than law he worships "Nature" (1.2.1). Then, holding up a letter he has forged, Edmund explains to the audience that he is plotting to steal the land of his half-brother, "legitimate Edgar" (1.2.17), by winning all his father, Gloucester's, affection.
Edmund, criticizing official legal order as unjust, decides to follow a more brutal "win or lose" natural order instead. At this point, a modern reader might be sympathetic to Edmund: it's not his fault he was born out of wedlock. The Edmund/Edgar sibling rivalry for paternal favor mirrors that between Lear's daughters.
As Gloucester returns from Lear's court, baffled by the events there, Edmund conspicuously hides the letter in his pocket. When Gloucester asks what it is, Edmund replies "no news […] nothing" (1.2.31-3). Gloucester cheerfully demands to see it: "the quality of nothing hath not such need to hide itself […] if it be nothing, I shall not need spectacles" (1.2.35-7).
The word, "nothing," repeated by Cordelia and Lear throughout 1.1, continues its echo here. Quickly falling for Edmund's tricks to turn him against his legitimate son, Gloucester displays a shortsightedness that matches Lear's own misjudgment of his daughter's.
Feigning hesitation, Edmund hands over the letter, explaining that Edgar sent it to him. Gloucester reads it aloud. The letter argues against the "aged tyranny" (1.2.53) that keeps sons enslaved to fathers past their prime. It goes on to hint that if Edmund will help Edgar dispose of Gloucester, Edgar will grant the bastard half of his legitimate wealth. Edmund adds that Edgar has often said that, with "sons at perfect age and fathers declined" (76-7), sons should take care of fathers as their wards.
In his forged letter, Edmund uses the kinds of criticisms of age that Goneril and Regan cited at the end of 1.1—and which, indeed, motivated Lear to give up his own power—to play on Gloucester's own anxieties and turn him against his other son.
As Gloucester grows enraged, Edmund pretends that he would like to urge moderation: he offers to approach Edgar about the matter, while Gloucester, in hiding, can watch. Gloucester agrees, saying that he would give up everything he has to know whether or not Edgar is actually so untrue to the "father that so tenderly and entirely loves him" (101-2). He adds that he has recently observed disorder in the skies that predicts all the chaos that has happened with Lear, Cordelia, Kent, and now him: "these late eclipses in the sun and moon portend no good to us […] we have seen the best of our time" (109-19).
Like Lear, Gloucester sees the heavens as both symbolizing and causing order or disorder in human affairs. He is incredulous that Edgar could violate the bonds of family. He thinks of these bonds not in Edmund's legal or economic terms of inheritance (because such issues don't affect him; he was a legitimate son of his father). Instead, he sees the bonds almost as a kind of rule of nature, embodying "tender" and complete emotional love.
After Gloucester has exited, Edmund mocks his father's belief in astrology: it is "excellent foppery," he says that when people suffer ill fortune, usually because of their own dumb behavior, they then blame "the sun, the moon, and stars" (125-8). Seeing Edgar, who has just then wandered in, Edmund briefly takes up the subject with him. Edgar is surprised at his brother's sudden interest in astronomy.
Edmund and Edgar both dismiss their father's faith in the heavens as being foolish and outdated (obliquely reflecting their age difference with their father, once again).
Then Edmund cuts to the chase, asking Edgar if he knows how he has offended Gloucester, who, Edmund reports, is enraged at his legitimate son. Edgar reacts with disbelief: "some villain hath done me wrong" (1.2.172). Replying that that's precisely what he fears, Edmund tells Edgar to go hide in Edmund's rooms, and advises Edgar that if he leaves his hiding place to make sure to carry a weapon to protect himself. Edmund promises to bring Edgar more news soon. Edgar rushes off.
Edmund, however, does continue to create just the kind of familial discord that Gloucester was troubled to observe in Lear's court and which, Gloucester predicted, were the result of the recent eclipses.
Once Edmund is left alone, he observes to himself that his father is trusting and Edgar is such a good person that he would never suspect someone else of being anything other than good. Dealing with such "foolish honesty" (1.2.189), Edmund says, will make it easy for him to take, through cunning, the lands that he did not inherit by birth.
Edmund explains that he will exploit the blind faith that a parent has in his child and the assumed trust between siblings in order to outwit the typical legal order (whereby the legitimate child inherits everything).