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.