An Ideal Husband is at times a difficult play to summarize as much of its "plot" happens through rapid-fire, epigrammatic dialogue. Indeed, the pace and subtlety of these turns-of-phrase are what make plot so easy to miss. When summarizing the story, one finds oneself paraphrasing the repartee (repartee is defined as a smart, ready, witty reply or a conversation distinguished by witty responses). To do so, of course, is to kill its wit and humor.

The following summary divides Act I into two parts, the first extending from the beginning to the fatal conversation between Sir Robert Chiltern and Mrs. Cheveley, the second starting from that conversation to the end. Act I opens with a dinner party given by Sir Robert in his fashionable Grosvenor Square home; thus it begins with a series of lighthearted conversations between the bantering guests. The entire act takes place in the brilliantly lit Octagon Room.

The first scene shows Lady Chiltern posed at the top of a large staircase greeting the guests. A tapestry representing Boucher's "Triumph of Love" hangs on the back wall. Sitting on a Louis Seize sofa, Mrs. Marchmont and Lady Basildon share the first conversation. Mrs. Marchmont declares that she has come to the soiree to be educated, and Lady Basildon replies that she abhors education. Mrs. Marchmont concurs but jestingly protests that their hostess—Lady Chiltern—is always urging her to find one "serious purpose" in life. Looking around the room—that is, both at the cast and audience—through her opera glasses, Lady Basildon notes that she hardly sees anyone whom one could call a serious purpose around here.

Act I similarly introduces its other players through such instances of banter, each new guest being announced by the butler Mason from the top of the stairs. Throughout this first scene, dialogue is occasionally punctuated by the malapropisms (the ludicrous misuse of words) of the Vicomte de Nanjac, a young anglophile whose awkward English serves as a comically distorted reflection of the group's polished repartee.

The following conversation involves Mabel Chiltern, Sir Robert's sister, and the elderly Lord Caversham, father of Lord Goring. Caversham bemoans the idleness of his son and the excesses of London Society. Along with introducing the old-fashioned Caversham, this conversation offers a glimpse at Mabel's affection for the dandified lord.

Suddenly Lady Markby and Mrs. Cheveley—sporting scarlet lips, a heliotrope gown, Venetian red hair, and rather assertive fan—enter the room. Upon their introduction, Lady Chiltern reveals coldly that she knows Mrs. Cheveley from their school days, and Mrs. Cheveley, having spent many years in Vienna, all too sweetly expresses her eagerness to meet Sir Robert. Lady Chiltern assures her she has little in common with her husband and moves away.