A first-person narrator sits in her living room smoking a cigarette on a January day after tea and looks into the fire. The sight of the fire draws her into reflections on the similarity between the coals and a cavalcade of knights. Seeking distraction from these thoughts, she catches sight of a black mark on the wall several inches above the mantelpiece. The narrator cannot immediately identify the mark, which provokes a sequence of reflections on its possible identity. She wonders if it might be a nail, a hole, a leaf, or something protruding from the wall. In between her various suspicions about the mark, she follows the flow of her consciousness on the topics of knowledge, the passage of time, subjectivity, and nature. She reflects deeply on the fleeting nature of life in modern civilization, as well as her identity as a woman and the impact of gender roles on contemporary society. Throughout the story, her thoughts circle around the ongoing war. The mark on the wall helps to ground her whenever her thoughts become too unpleasant.
Ultimately, a voice interrupts her reflections, revealing for the first time that she was not alone in the room. The voice states the desire to purchase a newspaper, remarks with distaste towards the ongoing war, and disparages the presence of a snail on their wall. The story closes with the narrator repeating to herself the realization that the mark had been a snail all along, disregarding the comments about the war.