Discovering a secluded signal box, a traveller observes a signalman staring towards the red danger light at the entrance to a railway tunnel. From his high vantage point, the traveller calls, 'Halloah, below there', waving with his right arm while placing his left arm across his face to shield his eyes from the sun. The signalman looks up at him in alarm. Although the signalman appears to be in dread of him, the travller reassures him that he is simply a man. The signalman had been wondering whether he had seen the traveller before, at the tunnel entrance, but the traveller denies this.

They talk in the signal box. The signalman's work taxes him not because of any strenuous effort it requires, but because of its responsibilities, on which he dwells. He tries to occupy his mind by studying mathematics, but he has little aptitude. He used to go up into the sunlight but his work drew him back down: the signalman describes his face being in sunlight while his mind is in shadow. He studied natural philosophy but became dissatisfied with the notion of everything having a reason.

The traveller awakes in the signal box at night, having fallen asleep. The signalman sees this as the trait of someone who lacks his serious responsibilities. Asked why he came to the signal box, the traveller says that he was drawn here. Discomforted by apparent vibrations in the warning bell, the signalman looks down across the railway line. He describes train crashes in tunnels, which are horrific because of the compressed space and fire and the screams of the injured and dying. The signalman agrees to meet the traveller the next night, but asks the traveller not to call out when he arrives. The traveller has a restless night at the local inn, suffering nightmares which include a tunnel collision, the warning bell and the request not to call out.

The next night, the signalman explains his concerns and why the traveller's first appearance disturbed him. He has experienced visions of a figure below the red light at the entrance to the tunnel, which calls out 'Halloah, below there' and repeating a gesture in which its left arm covers its face and its right arm waves as if to say 'For God's sake, clear the way'. Its first appearance a year earlier was followed within six hours by an accident in that tunnel, with the injured brought out past the point at which the figure had stood. The traveller suggests rational explanations for the noises and visions, and describes the incident as coincidence. However, around six months after that first incident, the figure appeared again. It stood, holding its face, then revealed a shocked, open-mouthed facial expression. At that moment, a train arrived, from which a woman was thrown, apparently killed, at that same spot. The woman's husband stood, holding his face.

The spectre returned a week before the traveller's arrival, and a further calamity seems inevitable. However, the signalman cannot report such a vague sense of danger and wonders why the spectre does not provide more information or warn someone with more power to act. The traveller reassures him that his diligence and sense of duty will suffice. That night, the traveller has another nightmare and awakes with a shocked, open-mouthed facial expression.

The next day, the signalman experiences another strange vibrating sensation from the warning bell, which has previously indicated the spectre's presence. Nearby, the traveller seems to experience it too, and runs toward the signal box. The signalman walks down the line to ask the spectre for information, in the process missing a signal to his box regarding an approaching train. The grey-haired spectre has a shocked, open-mouthed facial expression. The traveller arrives and calls out, but the train races through the tunnel and kills the signalman. The driver subsequently explains that he did not expect so diligent a signalman to be on the line, and had called out 'Halloah, below there', covering his face with his left arm while waving his right arm. The grey-haired traveller responds with a shocked, open-mouthed facial expression. Later, the traveller walks away into the fog.