The story begins with Jerry and his widowed mother on vacation from their native England to a coastal town in an unnamed foreign country. They seem to have visited the area many times before, as they already have a routine in place of visiting a certain popular beach. On the stroll down to this beach, Jerry notices the “wild and rocky bay,” set apart from their usual area and down a separate fork in the path. Partly out of a sense of adventurous curiosity and partly out of a desire to spend time away from his doting mother, Jerry sets off on his own to explore the rocky bay.
Already a strong swimmer, Jerry goes into the water and drifts far enough out that he can see his mother in the distance, just a small dot on the crowded beach. On his way back to the rocks, Jerry sees a group of local older boys who are diving and playing in the water. They motion for him to join them, so he does. Once the boys realize that he can’t speak or understand their native language, though, they ignore him. The biggest boy dives into the water and doesn’t come up for several moments. Jerry is surprised and yells out to the others, but they don’t seem concerned about the other boy’s disappearance. When the boy eventually surfaces in the water on the other side of a large rock, the rest of them follow suit and dive off the rock. Jerry goes in after them but can only see the surface of the rock. When they, too, suddenly reappear on the other side of the rock, Jerry realizes that they must have passed through an underwater tunnel.
As the boys prepare to perform the feat again from the diving wall, Jerry is desperate for their approval. He flails about and tries speaking to them in broken French, but they are unimpressed. One by one, the boys dive into the water and seemingly disappear. Jerry counts off the minutes, shocked at the length of time they are underwater. When he gets to one hundred and sixty, the boys reappear on the other side of the rock again and go back to the shore, ignoring him all the while. After Jerry returns to the diving rock, the boys leave to another area on the shore and he cries to himself.
Throughout the following days, Jerry spends all his time contemplating how he can get through the tunnel. He gets his mother to buy him a pair of goggles, he practices holding his breath, both underwater and on land, and learns to use a boulder to help sink himself into the opening of the tunnel. In his training process, he suffers nose bleeds and experiences nausea, starting to worry that this will happen to him as he is making his way through the long underwater tunnel.
When his mother says they’ll be returning back home in four days, Jerry decides that his opportunities to make his passage are disappearing and decides to make the attempt two days before they leave. When the day comes, Jerry employs all the tricks he has been practicing. When he is finally inside of the tunnel, his lungs start aching, his eyes burn, and he gets excessively lightheaded. A crack in the rock letting in the daylight gives him the illusion that his ordeal is over, but he is only partway through. When he eventually does make it through to the other side and emerges above the surface of the water, he is desperate for air and bleeding from a gash on his head, but feels elated at his accomplishment. Returning home, he sees the group of older boys, but feels no desire to win their approval any longer. He falls fast asleep when he gets home, and awakes when his mother returns. She asks about the gash on his head, but he doesn’t tell her of his courageous feat—only that he can hold his breath for over two minutes. She tells him not to overdo it, but he has no desire to return to the rocky bay again.
