In the quaint small town of Havana, Illinois, hydrologist Curtis Meyerson arrives to investigate reports of strange occurrences in the Illinois River. As he collects water samples, he notices an eerie blue mist hanging in the air, a sign of the impending crisis about to unfold. Meanwhile, at the Havana Police Department, Chief Larry Calhoun is left reeling after his own wife, Cari Lynn, a respected nurse, violently attacks their teenage son Robbie. Cari Lynn's paranoid delusions and frenzied behavior are just the first signs of a much larger problem gripping the town.
As Curtis continues his investigation, he witnesses a horrifying scene at the local park, where mothers have turned on their own children, attacking them with a terrifying ferocity. Dispatcher Patty fields a flood of desperate calls as fathers rush to the police station, their children injured by the very women meant to protect them. The small-town calm has been shattered, replaced by a pervasive sense of dread and the lingering question of what could drive mothers to harm their own flesh and blood.
Curtis rushes to his lab in Springfield, where his team, including his boss Mario and the imposing CEO of Axis Chemical, Theodora Hewson, makes a startling discovery. The blue algae in the river has evolved into a toxic gas that specifically targets biological mothers, turning them into violent, paranoid predators of their own children. The source of the contamination is traced back to Axis Chemical, who had improperly dumped chemical barrels into the river, setting off this unthinkable chain of events.
As the town descends into chaos, with mothers locked away and children huddled in the high school for safety, the military becomes involved, ready to take drastic action to contain what they now view as a bioweapon threat. Theodora, a Havana native herself, is torn between her corporate responsibilities and the desire to protect her hometown, even as Colonel Bechard presses for authorization to obliterate the town.
In a desperate race against time, Curtis works with the weathered pilot Kenneth Norfleet to modify crop dusters, planning a risky aerial mission to disperse a neutralizing chemical agent over the contamination zone. But with the affected mothers beginning to wake from their sedatives, the situation grows ever more volatile. Chief Larry faces an impossible choice-ensure the safety of the town by keeping everyone indoors during the spraying, or risk lives to try and protect the very women who have become a threat to their own children.
The crisis reaches a shattering climax when Larry is forced to make the unthinkable decision to shoot his own wife, Cari Lynn, to save their son Robbie. Traumatized by having to use lethal force against the woman he has loved since high school, Larry must now grapple with the aftermath, even as Curtis and his team prepare the experimental aerial remediation mission, praying it will restore normalcy to the devastated community. With the FBI investigation looming and the military poised to act, the fate of Havana hangs in the balance, a small town caught in the grip of a corporate-fueled environmental disaster that has torn families apart.