Walter Mitty becomes the most awesome person in the world in each of his "secret life" dreams. Walter Mitty is normal to a fault. He's essentially invisible to the people around him, so he imagines himself as an important, powerful, and central figure in each daydream.
He first imagines himself as a brave Navy pilot flying through a really bad storm. He is the only member of his crew that acts calm and brave.
Walter next imagines himself as a great surgeon that saves the day when an operation takes a turn for the worse.
A third day dream involves Walter on trial for murder. He didn't commit it, but he does admit that he is a good enough pistol shot to have done the murder.
Another daydream places Walter in the cockpit of a fighter plane. He is the last and lone help for the Allies.
Lastly, Walter's imagination places him in front of a firing squad.
In each case, Walter imagines himself as the hero of his own adventure story.