While Ribby is out shopping, Duchess slips into her house and places the veal and ham pie in the top oven in Ribby's kitchen. She searches the house for the mouse pie but is unable to find it; she does not suspect that it is in the bottom oven, the door of which is very stiff, and which she believed was only ornamental.
On her return home, Ribby notices evidence of an intruder in her house but cannot think who it could have been. When Duchess arrives for tea she is momentarily distracted and thus does not see Ribby taking the pie out of the bottom oven. Ribby gives the pie to Duchess while she eats muffins. Duchess becomes increasingly worried when she cannot find the patty-pan she placed in the pie (which she believes is her own), while Ribby denies ever using patty-pans at all.
Believing that she swallowed the patty-pan, Duchess feels quite ill and Ribby hurries out to find a doctor, Dr. Maggoty, a magpie. While she is away, Duchess finds her own pie in the top oven and realises that she is unwell because she has been eating a mouse pie after all. She hides her pie in the back garden, intending to take it home later.
After a lot of unwelcome fuss by Ribby and Dr. Maggoty, Duchess takes her leave, only to find that the magpie and a couple of jackdaws have eaten her pie of veal and ham.
Ribby later finds the remains of the pie dish and the patty-pan and decides that in future she will invite cousin Tabitha Twitchit for tea, even though she had considered her inferior company to Duchess.