On 4 March, the controllers of the Buenos Aires Underground discover that train UM-86—a Siemens-Schuckert Orenstein & Koppel model—has vanished along with its passengers in the network of tunnels. Despite an across-the-board search that, in the film, is shown as far larger than reality, neither the train nor its riders turn up. To keep the incident out of the public eye, the director of SBASE reaches out to the engineering firm responsible for the line, hoping for answers. The firm responds by sending a young topologist Daniel Pratt instead of a seasoned engineer, a choice that instantly displeases the director.
In pursuit of the original plans for the line, Pratt visits a national archive only to learn that the documents are missing, taken away by his former lecturer Hugo Mistein. He tracks down Mistein’s address and, after a detour at the university, discovers that the professor has not been seen for quite some time. Pratt then makes contact by phone and is directed to the professor’s home. When he arrives, he is greeted by the same uncooperative young woman Débora Vidret who answered the phone and who soon lets him inside.
The two head to a station to search for the director and the investigation’s backers. Pratt speaks to the officials who control the line, and they reveal that the entire route has been shut down to facilitate the search. As the group shares the grim update, the distant rumble of a train echoes through the tunnels. The director orders the power cut, a move that only fuels frustration and uncertainty among the officials. Pratt then unveils a bold, contentious theory: the perimeter construction has twisted the network into a kind of Möbius strip, a single surface that could send the missing train into another dimension under the right conditions. The panel dismisses his idea, and Pratt is relieved of his duties, walking away from the station with the girl in tow.
Later, after a visit to the Parque de la Ciudad roller coaster, Pratt’s mind begins to churn with calculations. He returns to the underground system with renewed zeal, recording his observations about train movements and schedules. He becomes convinced that UM-86 will reappear. He travels through maintenance passages and tunnels until he emerges at a different station named for the Argentine writer Borges. There, he boards the first train that arrives and climbs to the conductor’s cabin, where he finds Hugo Mistein waiting in the chair.
In a sequence that blends dialogue and reflection, Pratt and the professor discuss the validity of Pratt’s theory and critique aspects of modern society. The next day, the director of SBASE is summoned to a station where UM-86 stands empty, with Pratt’s journal left behind in the conductor’s cabin as the sole record of the investigator’s ideas. Just as the director prepares to depart, a phone call interrupts with news of another missing train, leaving the mystery unresolved and the film closed on a note of ongoing inquiry.
