The foreign, wild, and rugged setting advances the themes of alienation and maturation. This setting helps to advance the plot since the main character's actions and maturation are connected to the wild bay and its underground tunnel.
While the author provides no specifics regarding the setting, some literary critics have surmised that the "young English boy" and his mother are vacationing on an African coast since Doris Lessing herself lived in Rhodesia (known today as Zimbabwe). She once wrote,
Africa gives you the knowledge that man is a small creature, among other creatures, in a large landscape. [Enotes]
Because he is in a foreign country, Jerry has no friends with whom he can interact; his feelings of rejection at the hands of the French-speaking boys who swim away from him at the wild bay leave him feeling profoundly lonely. Alone, Jerry must wrestle with his feelings and make decisions on his own. He decides to figure out how the boys have reached the far side of the barrier rock because "to be with them, of them, was a craving that filled his whole body." When he is unable to imitate them as they go underwater through a tunnel in the rock, Jerry practices holding his breath until he can do so long enough to swim the length of the tunnel.
While he feels ready to make the passage through the tunnel, Jerry still undergoes some stress to his body. Nevertheless, he pursues his goal. As he exits the tunnel, Jerry feels victorious. Now "he wanted nothing but to get back home and lie down." No longer does Jerry desire being with the other boys: "he did not want them." He has undergone his rite of passage on his own in the rugged undersea setting.
Having accomplished his rite of passage on his own, Jerry feels independent. After he returns to where he is staying with his mother, she remarks that he looks a little pale. A more mature Jerry does not reveal what he has done; instead, he only tells her that he can stay underwater for two to three minutes, perhaps, so she will not worry.