An Ideal Husband 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 Gertrude Chiltern, are hosting a gathering that includes his friend Lord Goring, a dandified bachelor and close friend to the Chilterns, his sister 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, Baron Arnheim, convinced the young Sir Robert many years ago 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 both the ruin of career and marriage, Sir Robert submits to her demands.