Mercy Merrick has been tricked into the tragic life of a woman of the streets, after a childhood with strolling players and gypsies. She struggles to rehabilitate herself, inspired by a sermon given by a young clergyman, Julian Gray, in the refuge where she was living. Working as a volunteer nurse in the Franco-German war of 1870, she meets Grace Roseberry, a penniless but well-connected and respectable young woman. When Grace is apparently killed by a German shell, Mercy assumes her identity. Armed with a letter of introduction to Grace's relation by marriage, Lady Janet Roy, she is helped to reach England by Horace Holmcroft, a war correspondent and acquaintance of Lady Roy. Mercy is warmly welcomed and becomes her companion and adopted daughter. With Lady Roy's approval, she becomes engaged to Horace.