The tale of a prospective Cabinet Minister, Sir Robert Chiltern, who plans to expose a fraudulent canal scheme during a Parliamentary sitting. However, the mysterious Mrs. Cheveley arrives in Mayfair society and blackmails him into supporting it, under threat of exposing his past sale of government secrets to a foreigner. Lady Chiltern, ignorant of his reasons, refuses to accept his change of heart and insists he stand by his first decision. Honorable at heart, he writes to Mrs. Cheveley announcing his intention. She promptly reveals the truth to his wife, who threatens to end their marriage. But Chiltern's friend Viscount Goring recognizes a diamond necklace Mrs. Cheveley has left behind and realizes she must have stolen it from the lady he himself gave it to--giving him the material for a counter-blackmail bid to save his friend's marriage and career