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.
The plot of the story consists of the series of related events concerning Jerry, his mother, and the minor characters. The events develop one or more conflicts in the story. As these events unfold and the conflicts are developed, Lessing's theme--the truth or meaning of the story--takes shape and is finalized when the plot reaches its dramatic climax and the conflicts are resolved.
All of the events in the story lead us to believe that Jerry is immature and unsure of himself and that his mother treats him as a very dependent child. In her concern for his safety, she watches him closely. Jerry grows to chafe under her excessive protection. When Jerry meets the older boys at the "dangerous" beach, he humiliates himself and decides that he, too, will swim through the tunnel to show his strength and independence. The remainder of the story details Jerry's efforts to prepare himself for the dangerous swim and the ordeal when he does swim through the tunnel. When Jerry makes it through, his internal and external conflicts are resolved. He has won in his struggle to become strong and independent.
The theme of the story is made clear through the events as they transpire and their results. Jerry survives the tunnel and feels a great sense of satisfaction and self-acceptance. It would be reasonable, then, to conclude that one important theme in the story is that of growing up and achieving self-respect through personal courage and determination. We can reach that conclusion because that is what Jerry accomplished. Readers can interpret the events of a story in more than one way and find more than one theme.
whats thriugh the tunnel about