Edmund, leading the British forces with Regan, sends a messenger to Albany to confirm that Albany will send his forces to join theirs. Regan, meanwhile, pesters Edmund about whether or not he has ever slept with Goneril. Edmund tells Regan not to fear, just as Albany and Goneril approach. Goneril remarks to herself that she would rather lose the battle to the French than have Regan come between her and Edmand. Although Albany stresses that he is joining them against their common enemy of the French, not because he approves of their treatment of Lear and Gloucester, he and Goneril join Edmund and Regan. Edmund says he will join Albany shortly at his tent. Regan forces Goneril to walk off with her.
Goneril's obsessive jealousy of Edmund, and Regan's suspicion of Goneril, show the accelerating decay of their relationship and alliance. Albany, meanwhile, stresses that he is only temporarily suspending his principles voiced in 4.2 for the sake of maintaining the integrity of the British kingdoms. (He has not changed in his disapproval of the 'tiger daughters.')
As the others depart, Edgar, still in disguise as a beggar, approaches Albany. Edgar gives Albany the letter from Goneril to Edmund that he intercepted from Oswald, and tells Albany to read it before the battle. Then, Edgar says, if the British side wins, Albany must have a herald sound a trumpet and Edgar will appear again. Edgar exits. Just as Albany is starting to read, Edmund appears and hands him a report describing the strength of the French army. Albany takes it and exits, hurriedly.
By delivering the letter to Albany, Edgar will grant Albany full insight into his wife's character—and how her treachery will violate their marriage as well as her familial bonds. Edgar, meanwhile, is laying in place the plan to avenge himself on his brother.
Alone on stage, Edmund explains that he has sworn his love to both Goneril and Regan and muses about which it would be more convenient for him to marry. Resolving to leave that problem for the time being, he further reveals that, if the British are victorious, although Albany wishes to spare Lear and Cordelia, he never will.
Edmund's deceit of the two sisters reveals that they, too, are capable of misjudging. Edmund's malicious opportunism has an almost unlimited depth, even when compared to such treacherous people as Goneril and Regan.