Some years before the play begins, Mr. Posket, a London magistrate, married a widow, Agatha Farringdon. At the time she had pretended to be 31 rather than her true age, which was 36. Accordingly, she found that she needed also to knock five years off the supposed age of her son by her first marriage, Cis Farringdon. When the play opens, the Poskets are preparing to entertain to dinner the following day an old friend of Posket, Colonel Lukyn. The Colonel knew Mrs. Posket in her earlier days, and is Cis's godfather. He is well aware of Mrs. Posket's true age. Fearful that the Colonel may be indiscreet about dates, she slips out that evening to see him privately. She takes with her her sister Charlotte who is staying with the Poskets for a few days, getting over a broken engagement.Agatha's son Cis takes advantage of his mother's absence. Although he is supposed to be 14, he is in fact 19 without knowing it, and his precocity is far in advance of his supposed age. He smokes, he flirts, he gambles, and now, as soon as his mother has left, he coolly proposes to his staid stepfather that they should go to the Hôtel des Princes, where he keeps a room for the use of his friends. Posket allows himself to be persuaded.Greatly to his surprise Posket finds himself "making a night of it". Colonel Lukyn has also arranged to dine at the Hôtel des Princes, with his friend Captain Horace Vale, the man who has broken Charlotte's heart.. When Agatha and Charlotte arrive to see the Colonel, there is a lengthy reconciliation scene between Vale and Charlotte. Equally lengthy are the carousings of Posket and Cis in the adjoining room – so much so that a breach of the licensing laws is committed and the police arrive to search the house
The landlord, having put out the lights, brings all his law-breaking guests into one room and bids them conceal themselves as best they can. Posket and his wife hide under the same table, each unaware of the other's identity. When the police burst in, Posket and Cis make a dash to the balcony, which collapses under their weight, depositing them in the street below. The others are all taken into custody.Posket and Cis are chased by the police. Posket staggers back just in time to perform his magisterial duties at Mulberry Street court. He is tattered, bruised and dirty. He pretends to be much shocked when the chief clerk tells him that the first case he has to hear involves his friend Lukyn. Despite Lukyn's appeal, Posket permits no favours, and insists that the case must be tried in the normal way. On going into court, Posket is so shocked to find his wife in the dock that he finds himself, in a trance-like state, sentencing her to seven days' imprisonment without the option of a fine.Posket's excessive sentence of his wife and the other guests at the hotel is overruled on a technicality by Posket's fellow magistrate, Bullamy. Back at home, Posket feels the force of his wife's indignation, but she cannot avoid explaining her presence at the hotel, and the truth about her deception about her age comes out.