Moebius plot
"A train on the Buenos Aires subway system suddenly vanishes."
Buenos Aires, Argentina. The metro system has meanwhile become so large that it is a large complex cluttered tangle. One day an entire train appears to have disappeared without a trace. Sometimes people hear the train going through the dark tunnels, but no one sees it. The director of the metro is eagerly looking for an explanation for this whole event. The engineer in charge asks a friend of his, a mathematician, to investigate the matter. This man comes up with a possible explanation, but it's so unlikely that no one believes it...