The story takes place at sea, near the Gulf of Siam, and is told from the perspective of a young nameless Captain. The captain is unfamiliar with both his ship and his crew, having only joined their company a fortnight earlier. The Captain is furthermore unsure of himself, questioning his ability to fulfill the role of such an authoritative figure. These themes are explored through symbols throughout the novella.
The captain soon encounters a naked swimmer holding onto the side ladder of the ship while he is alone at night on look-out duty. He helps the mysterious swimmer onto the boat and hides him in his cabin without the rest of the crew's knowledge. He then learns of the mysterious swimmer's past; his name is Leggatt, and he swam away from a nearby ship, called the Sephora, where, as chief mate, he killed another crew member for insolence during a storm.
The captain keeps Leggatt hidden in his quarters, away from the suspicious crew members and a visit from the skipper of the Sephora. Eventually the Captain allows Leggatt to escape by bringing the ship perilously close to land for Leggatt to swim away safely, though this risky sailing maneuver nearly sends the ship into the rocks, testing the Captain's seamanship. He succeeds and leads the ship away.