Scene 1: Mrs. Halifax’s drawing room (Mrs. Halifax is a lady of whom characters often speak but who doesn’t appear in the opera.) Lord Puff, an old cat belonging to Mrs. Halifax, is surrounded by his friends from the Royal Society for the Protection of Rats (R.S.P.R.), a group of vegetarian pacifist cats. The cats await the arrival of Minette, a young country cat whom Mrs. Halifax has brought to London to marry Lord Puff. Arnold, Lord Puff’s only nephew, disapproves of the arrangement because he had hoped that his uncle would die a bachelor and leave him the entire inheritance. Babette, Minette’s sister, enters first to make sure that Lord Puff and his family are respectable enough for Minette. They meet with Babette’s approval. Minette enters and tells of her first impressions of London and the advice her minister gave her before her journey. Babette and Minette are dismayed to discover the strange attitude of Lord Puff and his fellow Society members toward protecting mice and rats. Louise, a little mouse adopted by the Society, tells how all of her family were eaten by cats before she was rescued by the Society. Minette, although very puzzled, decides to go along with her future husband’s behavior and stay in London.

Scene 2: Mrs. Halifax’s roof. At dusk, a love serenade is heard in the distance. Minette meets Tom, a young, attractive cat, who falls in love with her. Remembering the advice from her minister and her future as the wife of the president of the Society, Minette controls her feelings, but Tom throws himself at her feet, causing her to regret her recent engagement. Arnold, who was passing by, has hidden himself to observe the meeting. Tom is furious when he discovers he has been spied upon. Minette tries to calm him, then to convert him to the ways of the Society, but he runs away, heartbroken. Minette calls to him as the moon and stars appear.

Scene 3: Mrs. Halifax’s private chapel. As Minette’s wedding to Lord Puff is being prepared, Arnold attempts to stop it with the help of his moneylender, Mr. Jones. Disguised as a medical doctor, Mr. Jones tries to poison Lord Puff in order to help Arnold obtain his inheritance so the moneylender can finally receive that which Arnold owes him. When their attempt fails, Arnold reveals to everyone that a meeting between Minette and a strange young cat took place on the roof. Despite misgivings, Lord Puff decides to marry Minette immediately, since he is going to be president of the Society. The wedding is celebrated to the great despair of Tom, who attends disguised as a curate.

Scene 1: Mrs. Halifax’s drawing room. Minette, who is now a “London lady” whose hobbies are charitable works, music and the Society, practices her cello. Her sister, Babette, who is still in poverty, arrives to ask Minette’s help. Babette leaves after receiving some money, and Minette is once again left alone to practice her cello but is unable to concentrate because she increasingly thinks of Tom. Hearing him serenade outside her room, she believes the singing is a hallucination. However, she opens the window, and Tom, dressed in a soaked uniform, jumps into the room. He tells her that after seeing her married, he entered the army, brokenhearted. But at the moment of departing for a faraway colony, he realized that he couldn’t live without her. Madly in love, he throws himself at her feet. Just then, the door opens; it is Lord Puff and the members of the Society, who are shocked. Tom is arrested, and Lord Puff yields to his friends’ influence and asks for a divorce.

Scene 2: The Divorce Court. Before the Counsel of Prosecution, the Judge and jurors, Minette is accused by Arnold and Lord Puff. Tom is disguised as the Counsel of Defense, having locked the real lawyer in a closet. During an adjournment for tea, Tom makes himself known to Minette and explains to her how he miraculously escaped from punishment that could have been fatal. When the hearing resumes, he attempts to prove that Lord Puff has not consummated the marriage, but the real lawyer, who has succeeded m freeing himself, rushes in. Tom and Minette are found guilty. But the lawyer recognizes Tom as the long-lost only child of Lord Fairport. Heir to a vast fortune, Tom can now pay without any difficulty the penalty to which he has been condemned. He automatically becomes the commander of his army regiment, which he had deserted. Nothing seems to oppose his marriage to Minette.

Scene 3: Mrs. Halifax’s drawing room. Babette comes to take Minette to the country after her divorce but finds her tied in a sack. Minette, who had not been present when Tom’s real identity was discovered, had become despondent. Mrs. Halifax decided to end Minette’s sufferings by having her put to sleep. When Tom comes to announce that everything is well now that they are rich, he too is distressed to find Minette locked in a sack. There is no escaping the fate that Mrs. Halifax has decreed. Pitying poor Minette’s fate, Tom and Babette comfort each other. They immediately fall in love and ask Minette’s blessing for their future marriage. Tom throws himself at Minette’s feet for the last time and gives her his good-bye. The door opens; the ladies and gentlemen of the Society enter and demand that Tom give his fortune to the Society. He refuses and leaves with Babette. Minette, abandoned, waits for Mrs. Halifax’s servant to throw her into the Thames River.

Scene 4: The lawyer’s chambers. Tom, now the young Lord Fairport, has drawn up his will, designating Babette as his sole heir. As he is about to sign the document, the lawyer’s clerk, Lucien, stabs him from behind. Tom collapses as the door opens and the Society members enter. Having planned Tom’s death, the members now proceed to appropriate the Fairport inheritance. Minette’s spirit appears to the dying Tom for a last farewell. As a result of these experiences, Louise, the little orphan mouse, realizes she cannot trust the Society members; she decides to return to her old life as a naughty little mouse who spreads fear among ladies.