Charles Mason (1728-1786) and Jeremiah Dixon (1733-1779) were the British surveyors best remembered for running the boundary between Pennsylvania and Maryland that we know today as the Mason-Dixon Line. Here is their story as re-imagined by Thomas Pynchon, featuring Native Americans and frontier folk, ripped bodices, naval warfare, conspiracies erotic and political, and major caffeine abuse. We follow the mismatched pair--one rollicking, the other depressive; one Gothic, the other pre-Romantic--from their first journey together to the Cape of Good Hope, to pre-Revolutionary America and back, through the strange yet redemptive turns of fortune in their later lives, on a grand tour of the Enlightenment's dark hemisphere, as they observe and participate in the many opportunities for insanity presented them by the Age of Reason.