In a Chicago suburb, conflict flares: a reading by an LBGTQ+ author is canceled when the library hosting the event receives violent threats on social media and a single bullet is dropped in the outside book return. The police cannot guarantee safety at the event and the library regrettably cancels the reading. Christine Bernard, a retired nurse, rallies her friends to protest the book banning group in her village, and one morning while filling the bird feeders in her front yard, is gunned down and killed by three men in a passing car. They are arrested and linked to neo-Nazi and white supremacist ideology. The murder shocks the suburb and devastates Terry, the victim's husband.
He inherits his wife's friends and their activism, battling the homophobic, racist, anti-science, book banning, misogynistic, Christo-fascists trying to enforce their beliefs on the library, the school board, and the village council. Christine's friends, especially her progressive women's group, embrace the widower, acting as an impromptu support group to console him in his grief. His friend, Ferguson, also comforts Terry, and during a discussion one afternoon, when Terry vents his anger at the men who killed his wife, Ferguson dismisses the trio as pawns, telling Terry that if he wants to exact revenge, he should cut off the head of the snake, warning however. that the mythical hydra, when its head was severed, grew two back in its place. From that hypothetical conversation, Terry formulates a plan.
The group spearheading the book banning activities in the village, Guarding Our Families, is led by a religious zealot, Tom Prentice, and Terry decides to kill him, as a message to the man's supporters. To avenge his wife's death. Bearing in mind the lesson of the hydra, he also decides that if he gets away with Prentice's murder, he'll target others, beginning with a woman on the library board of directors, a Guarding Our Families ally, who a library volunteer in the women's group describes as Prentice's "useful idiot."
The decision to kill is less difficult for Terry then he expected. The planning tames his grief. He surveils Prentice, learns his habit of gathering at a local microbrewery every Friday night to watch baseball games and chat with friends. His walk home takes him through a park which affords Terry a concealed location from which to ambush the man. No guns, too noisy, Terry thinks. He selects a quicker method: knocking Prentice unconscious and then slitting his throat.
Throughout his planning, Terry reminisces about his wife and what cowards like Prentice have stolen from him by indoctrinating others, inducing them to do his dirty work. His wife, he knows, would condemn what he plans but he relies on what his friend describes as the Ferguson Rule: never put anyone in a place where they have nothing left to lose. Terry feels that every morning.
The Ferguson Rule is a character study of how a grieving 75-year-old widower addresses an issue confronting so many communities, analyzing the repercussions of political and religious violence, demonstrating how hate perpetuates a cycle of mutual destruction. Terry Bernard despairs for the future of his town and the nation but feels constrained by the Ferguson Rule, and the memory of seeing his wife dead on their front lawn beside a "Black Lives Matter" yard sign overrules his rationality. Terry admits as he plans the killing of Tom Prentice that the hydra will grow two heads to replace the one, but perhaps the next Prentice will hesitate, pause before spewing the hatred that induces violence.