Heisaku is a university student who develops an unusual obsession with observing his neighbor, Fumio, through the gaps in her curtains. His daily life consists of part-time work during the day, followed by evening classes and sometimes going out for drinks with friends afterward. However, his primary focus is a peculiar hobby: he is fascinated by interstices, the narrow spaces between buildings and through other openings, where he can discover discarded objects or watch details of people passing by. His favorite gap is the opening in Fumio's curtain, through which he spends considerable time peeping, driven by a desire to imagine who she is and what she does in her life. This habit is not portrayed as merely casual; he struggles with feelings of guilt and temptation related to his voyeuristic behavior.

In terms of personality, Heisaku is characterized by this strange and somewhat obsessive fixation, which speaks to a broader difficulty in forming direct interpersonal connections. His curiosity about Fumio is intense, leading him to construct fantasies about her based on the small glimpses he catches. What he does not initially realize is that his actions are not one-sided. Fumio is fully aware that he is watching her, and she has been observing him in return from her own window, even secretly taking numerous photographs of him going about his daily activities in his room. This mutual, secret observation forms the core of the story, as both characters are fascinated and obsessed with each other from a distance, yet neither takes the initiative to make contact in person for a significant period.

Heisaku s role in the narrative is as the central figure in this unconventional romantic comedy, which hinges on the strange love affair between a young man who spies on his neighbor and the woman who knowingly allows herself to be watched. His actions drive the initial premise of the story, as the plot follows the hidden game of watching and being watched. The key relationship in his life is, without question, the one he has with Fumio, despite the fact that it begins entirely through a crack in a curtain and from opposite sides of a window. This relationship is defined by a strange symmetry, where both are simultaneously the observer and the observed.

The primary development for Heisaku occurs when this indirect and secretive relationship is forced to change. The status quo is broken when the two finally encounter each other face to face in a real-world setting, specifically at the university cafeteria. This meeting brings an end to the purely observational phase of their connection and forces Heisaku to confront the person he has been spying on, bridging the gap between his private fantasies and the reality of a direct relationship. Regarding notable abilities, Heisaku does not possess any supernatural or extraordinary powers. His defining trait is his unusual and focused ability to find and observe through gaps, making him a voyeur whose entire world of romantic interest is mediated through these physical spaces.