"Once upon a time", Ribby, a "Pussy-cat" invites a little dog called Duchess to tea with plans to serve her "something so very very nice" baked in a pie dish with a pink rim. Ribby promises Duchess she shall have the entree entirely to herself. Duchess accepts the invitation but hopes she will not be served mouse. "I really couldn't, couldn't eat mouse pie. And I shall have to eat it, because it is a party."
Duchess has prepared a ham and veal pie in a pink-rimmed dish (just like Ribby's dish), and would much rather eat her own pie. "It is all ready to put into the oven," she says, "Such a lovely pie-crust; and I put in a little tin patty-pan to hold up the crust." She reads the invitation again and realizes she will have the opportunity to switch the pies when Ribby leaves on an errand.
Ribby has two ovens, one above the other, and she puts her mouse pie in the lower oven. She tidies the house, sets the table, and leaves to buy tea, marmalade, and sugar. Duchess meantime has left home with her ham and veal pie in a basket, passes Ribby on the street, and hurries on to Ribby's house. She puts her pie into the upper oven, and searches quickly for the mouse pie (which she does not find because she neglects to look in the lower oven). She slips out the back door as Ribby returns.
At the appointed hour, Duchess appears at Ribby's door and the party begins. Distracted for a moment, Duchess does not see which oven Ribby opens to remove the pie. Duchess eats greedily, believing she is eating her ham and veal pie. She wonders what has happened to the patty-pan she put in her pie, and, not finding it under the crust, is convinced she has swallowed it. She sets up a howl; Ribby is perplexed and annoyed but leaves to find Dr. Maggoty, a magpie.
Duchess is left alone before Ribby's fire, and discovers her ham and veal pie in the oven. "Then I must have been eating MOUSE! ... No wonder I feel ill," she muses. Knowing she cannot adequately explain her ham and veal pie to Ribby, she puts it outside the back door intending to sneak back and carry it home after she leaves for home. Ribby and Dr. Maggoty arrive and, after much fuss, Duchess takes her leave, only to find that the magpie (who has left by the back door) and a couple of jackdaws have eaten her ham and veal pie. Ribby later finds the remains of the pie dish and the patty-pan outside the back door and declares, "Well I never did! ... Next time I want to give a party – I will invite Cousin Tabitha Twitchit!"