The humor of "The Lumber Room" arises from the perceptions of a humorless, single-minded, imperious aunt set against the free-spirited, imaginative and impish mind of the boy Nicholas.
There is much that is autobiographical in "The Lumber Room" because Saki models the self-appointed aunt after a real-life spinster aunt named Augusta, who along with her sister Charlotte (called "Aunt Tom"), raised Hector, Ethel, and Charles Munro after their mother died. Ethel eulogized Augusta as a
...woman of ungovernable temper, imperious, a moral coward, possessing no brains worth speaking of, and a primitive disposition.
This "less delectable truth of human existence," as critics note, of the aunt's nature set against the whimsicality of childhood provide the situations in which the irrationality and dim-witted mind of the aunt is satirically contrasted with the ingenuity and creativity of Nicholas. For instance, Nicholas is well aware that his obtuse aunt will not look in his bowl when he says that a frog is in it because she has set it before him and, therefore, in her mind, there can be nothing unusual about it. Then, when she issues her peremptory order that Nicholas not enter the gooseberry garden, it never occurs to her limited mind that something might happen for which Nicholas might need to enter this garden, just as in her myopia and "primitive disposition" she neglects to consider any variables to the outings of the other children, assuming that they will have to enjoy themselves while Nicholas will have to suffer.
Later on, with impish cleverness, the ingenious Nicholas uses the dull aunt's nature as his weapon to defeat her when he feigns the obedient child who believes that he is being tempted by the Devil. Manipulating her own words against her, Nicholas refuses to come into the gooseberry garden, saying his aunt has forbidden him,
"Aunt often tells me that the Evil One tempts me and that I always yield. This time I'm not going to yield."
Then, when his aunt orders him to fetch the ladder, Nicholas sets up his trap--"Will there be strawberry jam for tea?"--since he is aware that the aunt will now agree so that she can be freed from the water tank in which she is confined. However, earlier, the aunt has declared that there was none when Nicholas knows that there is because he has looked in the cupboard himself.
"Now I know that you are the Evil One and not aunt," shouted Nicholas gleefully; "when we asked aunt for strawberry jam yesterday she said there wasn't any. I know there are four jars ...and of course you know it's there, but she doesn't, because she said there wasn't any. Oh, Devil, you have sold yourself!"
Nicholas walks away, enjoying the "luxury" of being able to talk to his aunt in this manner. Defeated, she later broods mutely over the ignominy she has suffered, while Nicholas delights in another flight of fancy about the story on tapestry in the secret lumber room.