Spirited fifteen-year-old horse lover Rachel Selby is determined to become a veterinarian, despite the opposition of her rigid father, her proper mother, and the norms of Boston in 1872, while that city faces a serial arsonist and an epidemic spreading through its firehorse population.
As a war between the humans and the Partials looms, Samm finds himself trapped beyond a toxic wasteland, and Kira finds herself under the control of Dr. Morgan who is desperate to save the Partials.