In the spring of 1776, thirteen-year-old Elspeth Culpepper wakes to the sound of cannons. It's the Siege of Boston, the Patriots' massive drive to push the Loyalists out that turns the city into a chaotic war zone. Elspeth's father--her only living relative--has gone missing, leaving her alone and adrift in a broken town while desperately seeking employment to avoid the orphanage.

Just when things couldn't feel worse, the smallpox epidemic sweeps across Boston. Now, Bostonians must fight for their lives against an invisible enemy in addition to the visible one. While a vaccine is being frantically fine-tuned, thousands of people rush in from the countryside begging for inoculation. At the same time, others refuse protection, for the vaccine is crude at best and at times more dangerous than the disease itself.

Elspeth, who had smallpox as a small child and is now immune, finds work taking care of Abigail Adams and her extended family as they await a turn at inoculation, but as the epidemic and the revolution rage on, will she find her father?