Doris Lessing's story has two settings; the "safe beach" on which Jerry's mother sits, and the "wild bay" where the native boys daringly swim through and underwater tunnel. It is in this wild bay in which Jerry undergoes the ordeal that changes him.
In the exposition of Lessing's "Through the Tunnel," the mother's arm can be seen, swinging loose as she and Jerry walk along. This arm, slightly reddened from the other day, swings and Jerry is tempted to run and accompany her. But, he does not accompany her to the safe beach. Instead, Jerry parts from the "speck of yellow" on the beach and swims out into the water. On the edge of the cape, Jerry spots some boys that are burned smooth dark brown and speaking a language he does not understand. Jerry wants to join them:
To be with them, of them, was a craving that filled his whole body.
After Jerry swims a little closer, they turn and watch him with narrowed, alert dark eyes, suspicious of his motives. When Jerry discovers that they have swum through an underwater tunnel, he perceives this ability to do so as the mark of a man. When Jerry returns to his mother on her beach, he insists that she buy him some swim goggles. Afterwards, he goes back to the area where the boys have plunged, and studies the area around this tunnel:
He knew he must find his way through that cave, or hole, or tunnel, and out the other side....On the day before they left, he would do it. He would do it if it killed him, he said defiantly to himself.
After much practice, Jerry tinally attempts the swim through the tunnel. It is a frightening experience as he feels his lungs ache and his head throb from bouncing against the sharp rock of the tunnel; however, Jerry is successful and no longer wants anything "but to get back home and lie down." For, now he is a man, Jerry feels, as he has made the passage from boyhood to manhood. For, by facing danger and overcoming it, Jerry has acquired greater maturity and independence.