In a future Boston, the subway system has evolved so that passengers can travel from any station to any other station, creating a highly-interconnected system of tracks and trains. On March 4th, passenger train Number 86 goes missing. After scanning the system, Kelvin Whyte, the general manager of the tracks, cannot find the train anywhere. He meets with famed Harvard mathematician Roger Tupelo, who explains the missing train must still be on the tracks because it is a closed system. Tupelo hypothesizes the topological properties of the system have made it possible for a train to disappear into the system itself. Whyte explodes, refusing to believe him. Later, Whyte apologizes and calls Tupelo back to the tracks, where multiple motormen recall hearing the missing train but not seeing it, confirming Tupelo's hypothesis. Tupelo asserts the train, though audible, cannot be retrieved unless it jumps from the second dimension back to the visible tracks on its own. Time passes and the missing Number 86 is soon forgotten. Months later, Tupelo takes the subway and notices his fellow passengers are reading old newspapers from March. He is on the missing train, which has returned to the tracks. The passengers do not know they have been lost. Tupelo stops the train and calls Whyte and the managers. After each passenger has been saved, Whyte informs Tupelo another train has gone missing.