Port Royal sits at the end of a narrow sand spit guarding Kingston Harbour, and by the late 1600s it is one of the largest English towns in the Americas, built on privateering and plunder. Sir Henry Morgan works out of the port, raiding Spanish treasure fleets along the Spanish Main, and the money that flows back turns the town into a byword for gambling, drink, and vice, the so-called Wickedest City on Earth. After 1670 the economy shifts toward the slave trade, sugar, and raw materials, and the harbor becomes a mercantile hub for the whole Caribbean. On the morning of June 7, 1692, an earthquake liquefies the sandy ground beneath the town, dropping whole streets into the sea and killing an estimated 2,000 people outright, with more dying of injury and disease afterward. The film follows the Institute of Nautical Archaeology's underwater excavations, run with Texas A&M and the Jamaica National Heritage Trust from 1981 to 1990, as divers map the drowned buildings and pull artifacts from a city that sank almost intact.