The episode covers a full year of work by the astronomers, opening in January 1764 with the reverberations of the massacre and the attack on Fort Pitt still being felt. Mason & Dixon make their base of operations south of the city near the house of John Harland. There, they erect a new base of observation and proceed to mark off the Tangent Line. The work occasions a discussion of the history of the boundary dispute and various mathematical and political attempts to set the line.