In a darkened room at dusk, a man in naval uniform rises from his knees beside a couch where a woman reclines. He has just declared his feelings to her during his five days of wartime shore leave. The woman breaks the silence by asking him to tell her a tale, the kind of professional story he used to tell before the war, set in some other world entirely. He reacts with surprise and irritation at what he perceives as her quick shift away from the emotional intensity they just shared. Before he begins, they exchange brief words about duty. He says he likes the word; she calls it horrible. He insists duty is not narrow but contains infinities, including "an infinity of absolution."
The narrator introduces two central figures: an unnamed Commanding Officer and a man known as the Northman, the master of a neutral vessel the officer will encounter. They inhabit a world at war across land, sea, air, and underground. The Commanding Officer's ship is a once-elegant vessel now repurposed for coastal patrol. He is struck by how unchanged the sea appears despite hidden dangers, particularly enemy submarines lurking beneath the surface. Night offers some relief from this deceptive calm, but fog creates an irritating illusion that one should be able to see but cannot.
One gloomy day, while steaming along a rocky coast, the second in command spots something floating on the water. The object itself is harmless, perhaps something like a barrel, but it is significant: The officers conclude it is evidence that neutral ships, those belonging to countries not officially participating in the war, have been secretly resupplying enemy submarines. They settle on the object having been left by accident during a hasty departure. The Commanding Officer is revolted by the murderous stealth and callous complicity that corrupt humanity's deepest emotions.
The woman on the couch interrupts, saying she understands the Commanding Officer's suffering. The narrator pauses, then continues. A wall of fog engulfs the ship in dense, motionless stillness. The officers navigate cautiously toward a small cove they know well and anchor blind. Shortly after, the Commanding Officer learns that another ship lies at anchor near the cove's entrance, barely visible through the fog. Both officers are struck that the other vessel made no sound as they entered, even though their leadsmen, the sailors calling out depth measurements, would have been audible at 50 yards. The Commanding Officer orders a boarding party.
The boarding officer reports the vessel is a neutral ship whose master told a complicated story of engine troubles, dangerous drifting, thick weather, and a decision to anchor along the coast. The engines are now operational with steam up, the cargo is harmless, and the papers are in perfect order. The boarding officer describes the master as a fine specimen of a Northman, civil but apparently recovering from heavy drinking, and denies him permission to proceed. The Commanding Officer and his second in command discuss whether this could be the ship that resupplied a submarine. The second in command warns they could never prove it. The Commanding Officer resolves to board the vessel himself, driven by the desire to detect what he truly seeks: the atmosphere of gratuitous treachery.
On the neutral vessel, the Northman leads the Commanding Officer to the stuffy chart-room and drops onto a disheveled bed-settee. He declares with intense earnestness that he does not know where he is, that fog pursued him for over a week before his engines broke down. He launches into a detailed account punctuated by brief, peculiar pauses. The Commanding Officer finds the tale more plausible than simple truth usually is, though he acknowledges this impression may be prejudice. The Northman adds that the ship is his own, mortgaged, and barely provides a living for his family. The Commanding Officer remarks pointedly that the Northman will make a fortune from the war. He tells the Northman his log-book confirms his story but adds that a log-book can easily be falsified. He wonders silently why the ship sat with steam up in the fog without making a sound, behavior he can attribute only to a guilty conscience.
Rather than confront the Northman directly, the Commanding Officer inspects the crew on deck. Their answers are consistent with the log-book, but he privately suspects they are a hand-picked lot unlikely to reveal anything. He catches himself alarmed that his suspicions are hardening into certitude without concrete evidence. Returning to the chart-room, he notices the Northman seems bolder and more glassy-eyed, suggesting he took another drink. He tells the Northman about the floating object and their conclusion about neutral ships resupplying submarines. The Northman inhales sharply, stands motionless as if thunderstruck, then produces what appears to be a fatuous smile. The Commanding Officer remarks that shooting is too good for those who abuse neutrality this way.
The conversation grows more charged. The Northman deflects blame to "the tempters," the people who offer bags of gold to vulnerable neutral captains. The Commanding Officer flatly denies having suspicions, and at that precise moment feels an inner certitude of the Northman's guilt. Apparently relieved, the Northman speaks more openly, mentioning he was recently in Rotterdam, a major Dutch port. He describes in hypothetical terms how a man with nerve could smuggle submarine supplies disguised as ordinary cargo, but insists he himself lacks the nerve and would go crazy from anxiety or take to drink.
The Commanding Officer declares that the punishment should be death, then announces he is clearing all neutral vessels off the coast. This ship must leave in half an hour. The Northman protests that he cannot leave in such fog, repeating that he does not know where he is. Seized by fury, the Commanding Officer gives him a specific course: steer south-by-east-half-east for about four miles, then haul eastward. The Northman resists but submits, panting that he has had enough. The Commanding Officer returns to his own ship and tells his officers, "Yes, I let him go" (27). He hears the neutral steamer picking up anchor and steaming out.
The narrator breaks out of the tale's frame, leaning toward the couch. He confesses that the course he gives would lead the Northman straight onto a deadly ledge of rock. The Northman strikes the ledge and goes down with all hands. Dropping all pretense, the narrator identifies himself as the Commanding Officer. He reasons that the sinking proves the Northman told the truth about not knowing where he was, but it may have been the only truth in an otherwise false story. He gives that course as a supreme test, and at the time he feels certain of the Northman's guilt. He declares that he does not know whether he carried out stern retribution or committed murder, whether the men who drowned were completely innocent or basely guilty, and that he will never know.
He rises. The woman gets up and throws her arms around his neck, knowing his passion for truth, his horror of deceit, his humanity. She begins to speak with compassion, but he cuts her off, repeating sternly that he will never know. He disengages, presses her hands to his lips, and walks out.