"The Red Convertible" tells the story of two brothers, Lyman and Henry, who more deeply bond with each other over the ownership and usage of their red convertible. Henry is pushed into a war that is likely filled with unbelievable horrors. He returns a completely changed person, and Lyman takes it upon himself to figure out a way to mentally heal his brother. Lyman decides that using the car is his best option; however, his efforts do not matter. Henry is such a changed person that readers are left to question whether or not he intentionally killed himself at the end.
Within this story are various themes, but the question asks how the theme drives the plot. This is an interesting way to look at themes, because it makes a reader consider that an author decides on specific themes and writes a plot around those themes. If the intention was to explore a theme on loss of innocence or the realities of being an adult, then a story is needed that forces characters to grow up very quickly through traumatic experiences.
Having a character go to war is a good way to have a character quickly move from an innocent child to a shell-shocked adult. By having Henry go through this change, Lyman is also quickly forced into the realities of adulthood by actually having to lose his brother multiple times. Lyman loses Henry physically to the war. Then Lyman learns that he has lost Henry both emotionally and mentally as a result of the war. Finally, Lyman permanently loses Henry after Henry drowns.