In "Through the Tunnel" by Doris Lessing, how does the plot relate to the theme?

There are several themes in the story that are related to the plot itself.  Plot usually drives the theme; it is the text of thematic content.  One theme in this story is that of loneliness and alienation.  To emphasize this theme, Lessing has a character in-between childhood and adulthood, who is fatherless, with a polite but tense relationship with his mother.  She emphasizes his loneliness when he sees the foreign boys swimming and to be playing with them is a "craving that fill[s] his whole body."  He wants companionship so badly, and we see that through the other boys.  But, he doesn't seek that belonging with his mother, he looks elsewhere, and when the boys leave him, he is prompted to take on the tunnel.

Another theme is the idea of rites of passage; boys going through tests and challenges that prove they are ready to become an adult, a man, and leave childhood behind.  The plot has Jerry first of all leaving the safe beach to explore-taking on a more adult challenge.  Then, as he trains and practices for the tunnel dive, we see him pushing his limits, leaving childhood play behind, and initiating rites of his own that he feels he must pass.  As he succeeds, he becomes more independent from his mother and confident in himself as a separate entity from her.  He feels matured and responsible, and capable of taking on challenges.  Lessing implements the tunnel challenge to correlate with that theme of achieving adulthood.