In a somewhat autobiographical portrayal, Saki depicts Nicholas as a bright, imaginative, clever boy, possessive of much, reason, wit, and talent for pranks.
Oppressed by the unimaginative soi-disent [self-proclaimed] aunt, who is really not his relative, the intelligent and highly imaginative Nicholas resents her authority. In an act that defies her position of superiority, he places a frog in his basin of bread-and-milk to prove that she is presumptive in her judgment that because she is an adult, she is superior in intellect to him. For, she declares him a liar for saying that he has a frog in his breakfast because of her adult omniscience:
...the fact that stood out clearest in the whole affair, as it presented itself to the mind of Nicholas, was that the older, wiser, and better people had been proved to be profoundly in error in matters about which they had expressed the utmost assurance.
In her humiliating defeat in proving her judgement wrong, the aunt retaliates by denying him a trip to the Jagborough beach. However, Nicholas again revenges himself as he informs her with a "grim chuckle" that
"Bobby won't enjoy himself much, and he won't race much either...his boots are hurting him. They're too tight."
When the aunt seeks to diffuse her responsibility in this matter by saying that Bobby did not say anything, Nicholas points out her obtuseness because he did, indeed, tell her but she failed to listen. So, in retaliation for this defeat, the aunt forbids Nicholas to go into the gooseberry garden. Saki's satire of the mindless manner of the upper crust aunt is evinced in her reply to Nicholas's inquiry as to why he cannot enter this garden: "'Because you are in disgrace,' said the aunt loftily."
"[A] woman of few ideas, with immense powers of concentration," the aunt assumes that the defiant Nicholas will slip into the garden, so she carefully watches it in order to be able to punish him further." However, the clever Nicholas, who knows that she will be so attentive, defies her in another way by stealing the key to the lumber room, a storage room forbidden to the children. There, he satiates his imaginative spirit much more than a trip to Jagborough would have. For, there are all sorts of items that incite his imagination in this "storehouse of unimagined treasures" of a tapestry of a hunter who is watched by hungry wolves, a duck-shaped teapot, brass figures, and an interesting and colorful bird book.
Of course, his great retaliation for his aunt's insensitivity to him and dictatorial behavior arrives when she slips and falls into the rain-water tank. With a certain sarcasm, Nicholas informs her that he cannot help her since she has forbidden him to enter the garden. Further, when the aunt countermands her original order, Nicholas turns her accident into an ingenious prank as he pretends that he does not think she is truly his aunt,
"Your voice doesn't sound like aunt's,...you may be the Evil One tempting me.... Aunt often tells me that the Evil One tempts me and that I always yield This time I'm not going to yield."
So, in ridicule of her oppressiveness of yesterday and folly of today, Nicholas asks her for strawberry jam, and she agrees. She cannot, then, be his aunt, Nicholas claims, since his aunt declared there was none yesterday: "Oh, Devil, you have sold yourself!"