"The Secret Life of Walter Mitty," first published in 1939, is a captivating short story that delves into the world of daydreams and the transformative power of imagination. In just a few pages, this tale follows the mundane life of Walter Mitty, an otherwise ordinary man who escapes his humdrum reality through vivid and daring fantasies. 

As the narrative unfolds, author James Thurber explores the contrast between Mitty's unremarkable everyday existence and his larger-than-life daydreams, shedding light on the human desire for adventure and self-discovery.

Walter Mitty, a meek and unassertive individual, is married to his strong-willed wife, Mrs. Mitty. He is frequently henpecked by her and constantly chided for his forgetfulness and perceived incompetence. 

His wife seems aware of Mitty's tendency to drift off. However, she clearly has no appreciation for his behavior. In fact, she seems to think of it as some sort of illness. At one point, she says to him,

It's one of your days. I wish you'd let Dr. Renshaw look you over.

Rather than encouraging Mitty to visit the doctor, this comment sends him daydreaming of being a respected physician that even Dr. Renshaw looks up to.

Throughout the story, Mitty embarks on a series of errands and tedious tasks, such as driving his wife to the hairdresser and shopping in town. However, the narrative's driving force lies in Mitty's tendency to drift into his imaginary worlds at the slightest trigger – be it a sight, a sound, or a thought.

In these elaborate daydreams, Mitty transforms into a variety of daring characters, displaying exceptional skills and the courage he lacks in reality. In his imagination, he always envisions himself as a confident and assertive person, a foil to his actual self. In each fantasy, he faces perilous challenges with dashing flair and bravado, reflecting his inner psyche's unfulfilled desires and repressed yearnings.

Amid his colorful daydreams, the story's tone subtly shifts to reflect the real world's starkness. Each time Mitty returns to reality, he is confronted with mundane and disheartening circumstances, reminding him of his ordinary life. Whether it's a parking attendant telling Mitty to "Back it up, Mac! Look out for that Buick!" or his wife scolding him for not sitting in a more conspicuous place, these moments are always a disappointment. 

Nevertheless, as the narrative progresses, these two parallel worlds start to merge, blurring the boundaries between fantasy and reality. The narration often slips in and out of Mitty's fantasies so smoothly that the reader is momentarily left wondering what is reality and what isn't. 

This technique is expertly achieved in the opening of this short story, in which Mitty imagines himself as the commander of a naval hydroplane. It isn't until Mitty's wife interjects to tell him that he is driving too fast that the reader understands that this is merely a fantasy of the story's protagonist. 

Very little happens to Mitty during the story. He drops his wife at the hairdresser, buys some overshoes and dog treats, and waits in a hotel lobby. He also struggles to complete several ordinary tasks, such as parking the car and remembering the correct brand of dog food. Contrast this to his fantasies in which he is everything from a war hero to an overconfident defendant on trial. These over-the-top daydreams are as fanciful as his reality is dull. 

The story ends with Mitty seemingly accepting his unexciting life but in the form of yet another rebellious daydream. In the final paragraph, Mitty sees himself as a condemned man stoically facing execution. 

Then, with that faint, fleeting smile playing on his lips, he faced the firing squad; erect and motionless, proud and disdainful, Walter Mitty the Undefeated, inscrutable to the last.