What does Huck blurt out that makes it impossible to continue the fraud any longer? CH 24-28

Mary Jane explains that she is upset about the slaves being so mistreated, and Huck blurts out that they will be together again in two weeks at the most, knowing the Duke and King will abandon the town. When he realizes he has slipped, he decides to tell her everything. She becomes furious as he relates the story, and when Huck finishes, she calls the King a "brute."