Eddard Discovers the Truth

Eddard Stark, through his investigations, learns that Cersei Lannister's children are not Robert's but are the product of her incest with Jaime. He confronts Cersei in the godswood, giving her a chance to flee with her children before he tells Robert. This revelation is the central secret of the novel and sets the stage for the succession crisis. Eddard's decision to confront Cersei rather than go directly to Robert proves to be a fatal mistake.