Booker's discussion of comedy doesn't include the same easily-discernible stages of his other type of plots. Instead, he examines a few different types of comedies and the typical traits they feature. The one that caught our eye when thinking about "Walter Mitty" is Booker's third type, or new, comedy. In this type of comedy, humor stems from some sort of misconception or misunderstanding of reality. The characters are not fully conscious of the truth, Booker says.
In this case, it seems that our hero Walter Mitty is intentionally obscuring the truth from himself. His fantasies involve a rejection of everyday reality, and so his imaginings hide the truth from his eyes. He is actually just a bumbling, aging man – not a pilot or commander or surgeon – but he doesn't want to recognize that.
The end of the classic comedy comes about when the truth comes to light: everyone finally figures out what's really going on. This is where "Walter Mitty" decidedly does NOT fit the classic expectations. Walter remains in his fantasy world, and his wife remains clueless as to her husband's feelings and aspirations. And this is, indeed, where the bittersweet or even tragic element of the story comes in. It would have been a much lighter tale, indeed, had Walter stopped his fantasizing and returned home in his little car decidedly under the speed limit all the way. But his refusal to do so hits home, in an emotionally resonant way. Whether this renders "Walter Mitty" a tragic tale is, however, subject to debate.