The Great Mouse Plot is a trick Roald Dahl and his friends play on the shopkeeper Mrs Pratchett.
One day Dahl and his friends find a stinking dead mouse in the hiding place they use for their sweets and other treasures under the floorboards in their classroom. They are about to throw the mouse out of the window when Thwaites has an even better idea. He suggests that they put the mouse in one on Mrs. Pratchett's sweetie jars. "Then when she puts her dirty hand in to grab a handful, she´ll grab a stinky dead mouse instead."
Feeling "like a gang of desperados," they enter Mrs. Pratchett's shop and Thwaites asks for a sherbet sucker and a bootlace. As Mrs. Pratchett's back is turned, Roald Dahl takes off the lid of the gobstoppers jar and drops in the dead mouse. As Roald Dahl says, "I felt like a hero. I was a hero. It was marvellous to be so popular."