The story starts with a daydream - Walter imagines himself to be Lieutenant Berg, commander of an aeroplane, and plunging into the depths of a hurricane. We then switch to real life and Walter Mitty's wife telling him off for driving too fast.
Then, Mitty plays an expert doctor with his name, who is just about to perform an incredibly difficult operation on a millionaire banker. When disaster hits, Mitty the doctor is able to save the day. Then we switch back to Walter Mitty driving his car and buying overshoes.
Walter then again plays himself but in the witness stand being accused of murder by a district attorney. He calmly boasts that he would have been able to carry out the murder and then is jerked back into real and mundane life, buying puppy biscuits!
While waiting for his wife to finish at the hairdresser, he becomes Captain Mitty, a wartime pilot, about to embark on a dangerous mission. Interrupted by his wife, they head back together, and whilst his wife picks up something from the drug store, Walter Mitty is transported in front of a firing squad, denying the option of having his face covered as he faces his last few moments....
Hope it helps!