In this daydream, Walter Mitty is a very famous doctor. (The daydream is triggered when he drives by a hospital.) He is helping a couple of other famous doctors who are doing some sort of surgery on a very important patient (a friend of President Roosevelt). Not only is Mitty asked to help, he is also called on to save the day by fixing a machine that is breaking down (it gives out anesthetic).
He is wakened from the daydream by the attendant at the parking lot. Mitty has been driving his car into the lot by the exit only lane.