In Act 1 of King Lear, Lear decides to divide his kingdom among his three daughters based on their declarations of love, leading to his disownment of Cordelia, who only offers a genuine, modest expression of love. Meanwhile, the illegitimate son Edmund plots against his legitimate brother Edgar, while Goneril and Regan begin to conspire against their aging father, revealing their true natures. The act highlights themes of betrayal, the consequences of vanity, and the complex dynamics of familial relationships.