Cut off from the Line, what will Signy Shepherd do when the very people she protects become more dangerous than the threats they're escaping? Signy Shepherd has spent her career with the Line, a modern underground railroad, shepherding at-risk women out of peril. When Signy takes Lizzy, a young woman desperate to save her infant son, under her protection, the case appears to be like any other. With a severe winter storm on the horizon, Signy drives Lizzy and her son out of the city. Suddenly, she finds the police hot on their tail, and when Lizzy's erratic behavior propels them into further danger, Signy begins to suspect that her new ward is not the victim she claims to be. Meanwhile, Signy's PTSD-stricken mentor, Grace, investigates Lizzy's husband. But Lizzy's husband is hiding secrets of his own, and soon Grace finds herself out of her depth. As the treacherous blizzard closes in, the entire operation spirals out of control. Isolated and relying on nothing but her instincts, Signy is confronted with a choice that will force her to risk not only her own life, but those of the people she cares about most.