The year is 1917. Penny and her little sisters, Emily and Maggie, live with their father in a small house in Halifax. The girls' mother died giving birth to Maggie the year before. Her father, a building contractor, struggles with the challenges of raising three daughters, and plans to send Penny off to Montreal to live with her grandmother and aunt, in the belief that she would be better off raised by women - a notion Penny strongly resists.

On the morning of December 6, Penny's father is at work, leaving Penny to get her sisters ready for the day. It is then that a catastrophic explosion rocks Halifax harbour - and Penny, alone, must save her sisters' lives as well as her own.