I think we all understand what is meant by the plot of a novel. It is the story line; the summary of what happens. The theme is the message that the author is trying to get the reader to think about. It is the philosophical/theological/social/psychological message of the novel. The theme may not be very clear; it may be quite subtle or implied, because the author wants to present the reader with a puzzle: something important to consider.
It is probably fair to say that every novel has a plot, but not every novel has a theme. For example, my novel, The Iranian Scorpion, is a thriller, and as such, it has a plot, but I didn’t intend it to have a theme. I suppose, considering the novel retrospectively, one might say that its theme is the near impossibility of banning addictive drugs such as heroin, but I didn’t intend to write the novel to make that point.
Consider To Kill a Mockingbird, one of the great novels of the 2oth century. The plot is quite complex. It involves two young children, Scout and Jem, who live with their widowed father, a lawyer, in a small Alabama town in the 1930’s. The father, Atticus, is appointed by a judge to defend a black man who is accused of raping a white woman. In the course of the trial, Atticus establishes that the white woman and her father are lying. Nonetheless, the black man, Tom Robinson, is convicted by the jury. Tom is killed in escaping from jail. What follows is an attack by Bob Ewell, the accused’s father, on the children at night. Boo, an elusive and mysterious neighbour, intervenes. Bob Ewell is thought to have fallen on his own knife and died. The plot itself has elements of uncertainty: the evidence presented at trial, the attack on the children, the motivation of Boo.
The overriding theme of the novel is the racial prejudice which existed in the American South in the ’30’s. But there is also the idealistic courage of Atticus and his children in the face of prejudice. In addition, there are issues around social class and gender which are touched on.
I think it is fair to say that the plot, while it reflects some of the author, Harper Lee’s, childhood experiences, is constructed so as to develop the themes for the reader. Harper Lee took two and a half years to complete the novel, and during that time, she became so frustrated that at one point she threw the manuscript out a window into the snow. (Her agent made her retrieve it.) In my view, To Kill a Mockingbird is the best example of compelling plot and themes beautifully integrated.
A lesser example would be my novel, Sable Shadow and The Presence, which has as its themes the overriding importance of identity for us as human beings. Identity is who, why and what we are. It is critical in determining how happy we are in the life we lead, and our identity can be changed under certain circumstances. The plot is the life of a bright, but introverted male character who grows and develops into a ‘great success’, only to see his success evaporate, and having to build a new identity.