 Buenos Aires. Argentina. The subway system has been expanded over and over. It is that complex now that nobody is able to picture it anymore. Some day a whole train disappears. From time to time it can be heard rushing through the dark labyrinth, but it cannot be seen. The director of the subway must come up with an explanation for this phenomenon and sends for the responsible engineer. The engineer does not want to see the director and sends a friend, a mathematician, for investigation. Soon the young man comes up with the only possible thesis what may have happened to the train. Anyway the thesis is so incredible that nobody can believe it.