The play opens during a dinner party at the home of Sir Robert Chiltern in London's fashionable Grosvenor Square. Sir Robert, a prestigious member of the House of Commons, and his wife, Lady Chiltern, are hosting a gathering that includes his friend Lord Goring, a dandified bachelor and close friend to the Chilterns, Mabel Chiltern, and other genteel guests. During the party, Mrs. Cheveley, an enemy of Lady Chiltern's from their school days, attempts to blackmail Sir Robert into supporting a fraudulent scheme to build a canal in Argentina. Apparently, Mrs. Cheveley's dead mentor and lover, Baron Arnheim, convinced the young Sir Robert to sell him a Cabinet secret, a secret that suggested he buy stocks in the Suez Canal three days before the British government announced its purchase. Sir Robert made his fortune with that illicit money, and Mrs. Cheveley has the letter to prove his crime. Fearing the ruin of both career and marriage, Sir Robert submits to her demands. When Mrs. Cheveley pointedly informs Lady Chiltern of Sir Robert's change of heart regarding the canal scheme, the morally inflexible Lady Chiltern, unaware of both her husband's past and the blackmail plot, insists that Sir Robert renege on his promise. For Lady Chiltern, their marriage is predicated on her having an "ideal husband"-that is, a model spouse in both private and public life that she can worship: thus Sir Robert must remain unimpeachable in all his decisions. Sir Robert complies with the lady's wishes and apparently seals his doom. Also toward the end of Act I, Mabel and Lord Goring come upon a diamond brooch that Lord Goring gave someone many years ago. Goring takes the brooch and asks that Mabel inform him if anyone comes to retrieve it. In the second act, which also takes place at Sir Robert's house, Lord Goring urges Sir Robert to fight Mrs. Cheveley and admit his guilt to his wife. He also reveals that he and Mrs. Cheveley were formerly engaged. After finishing his conversation with Sir Robert, Goring engages in flirtatious banter with Mabel. He also takes Lady Chiltern aside and obliquely urges her to be less morally inflexible and more forgiving. Once Goring leaves, Mrs. Cheveley appears, unexpected, in search of a brooch she lost the previous evening. Incensed at Sir Robert's reneging on his promise, she ultimately exposes Sir Robert to his wife once they are both in the room. Unable to accept a Sir Robert now unmasked, Lady Chiltern then denounces her husband and refuses to forgive him.