It was in the middle of a street fight during the dismal winter of 1775 that Margaret Knox first spotted Islay McInnes, a Highland girl struggling to make a living for herself and her dependants among the Paisley slums. At sixteen, Islay is already the head of her household and Margaret - a charity worker and teacher at the local school - sees it as her Christian duty to ease the unfortunate girl's plight. Margaret finds new accommodation for Islay above the shop of her friend Jamie Todd, but soon, as well as dramatically changing Islay's and Jamie's life, Margaret's benevolent spirit brings confrontation into her marriage. Her husband Gavin is a well-respected surgeon, and though they share a deep concern for the health of the community, Margaret does all she can to prevent her beloved husband intervening in God's work. For Gavin desperately seeks a cure for the pox that threatens not just the poor children of Paisley but also his own...