The narrative begins with the protagonist, Sasayama Jo, a medical student, encountering a mysterious, fair-skinned young man with striking amber eyes at a train station. The man, who speaks fluent Japanese, warns Jo that the island platform is unsuitable for suicide due to train deceleration curves. He introduces himself as Ricard and mentions he is staying at a hotel near Jo's station, hinting at a potential future meeting. Jo, intrigued by Ricard's enigmatic aura and unusual demeanor, misses his train in his eagerness to learn more about him.
Ricard's past is then revealed through his own narration, detailing a lonely childhood in Lotford, England. Afflicted by asthma and a chest illness, he was sent to a lakeside villa with his nanny and a cruel servant boy named Nigel. Despite the beautiful natural surroundings, Ricard felt isolated and threatened by Nigel's taunts about ghosts. He recounts a pivotal night where a mysterious man in a long coat beckoned him from the lake. Driven by an inexplicable excitement, Jo followed, leading to a boat ride on the lake with the man, who introduced himself as Ricard. They bonded, and Ricard promised to be friends, claiming to know everything. This encounter, initially dismissed as a dream, is confirmed by a dirty towel left behind.
Jo's medical school life is depicted through his interactions with his study group: Aoyama, Shindo, and Mamiya. During a surgery, Jo experiences a profound existential crisis, contemplating the act of killing and the fragility of life, which leads to a panic attack and a diagnosis of pneumothorax. This event highlights his detachment from his peers and his internal struggles, exacerbated by his father's recent suicide and his mother's overbearing concern.
Jo's fascination with Ricard grows, and he begins visiting Ricard at his hotel. Ricard, a man of immense knowledge and a detached yet profound presence, discusses Natsume Soseki's "Kokoro" with Jo, offering a unique interpretation of the characters' motivations. Ricard's past is further explored, revealing his departure from Lotford with Ricard, a journey that takes them to 19th-century Edinburgh. There, they live in a small room in the old town, with Ricard working and returning late, his activities shrouded in mystery. Jo's health deteriorates in Edinburgh's poor air, but Ricard cares for him, claiming it's because they are friends. Ricard also reveals the existence of a hidden underground city in Edinburgh, a place of suffering and death, which he claims to hear.
The narrative then shifts to Jo's personal life, revealing the trauma of his father's suicide and his strained relationship with his mother. His mother's constant calls and his own emotional distance create a palpable tension. During a heated phone call, Jo confronts his mother about his father's death, accusing her of contributing to it, which leads to her hanging up. This emotional turmoil, coupled with his medical studies and the lingering mystery of Ricard, weighs heavily on Jo.
The story culminates with Jo's realization that Ricard's seemingly detached demeanor is a facade for deep-seated loneliness and a desire to protect him. Ricard's cryptic farewell and his departure leave Jo with a profound sense of loss and a deeper understanding of his own isolation. The narrative concludes with Jo reflecting on his past, his relationships, and the enduring impact of Ricard, suggesting a future shaped by these complex encounters.