1752, the Liverpool Merchant sets sail from Merseydale. A slaver, she is bound for Africa to buy men and transport them in chains across the Atlantic. But aboard the ship disease thrives in the cramped hold, killing men and eating into profit. Captain Thruso insists on throwing the sick overboard - the the horror of Matthew Paris, ship's surgeon, who determines to prevent such barbarity. Meanwhile, back home, Erasmus Kemp, cousin of Paris and son of the Liverpool Merchant's owner, finds his fortune hanging in the balance: depending on the success or failure of a single voyage.