Fortress

Sally Jones runs a one-room school in the Outback with nine pupils. That ordinary day, the younger children claim to have seen cartoon characters at the edges of the playground. Sally writes off the reports as childish fantasy, setting an uneasy tone for what comes next.

Four men wearing Halloween masks burst into the school, led by a man Sally nicknames Father Christmas. They force Sally and the children into a van at gunpoint, taking control of the classroom. Sally remains calm and tries to keep the kids cooperative.

In the van, the kidnappers openly discuss holding the school for ransom and threatening to murder one child a day until demands are met. Sally pretends innocence and asks for a rest break, while secretly ordering one of the older kids to hide in the woods to seek help. The kidnappers immediately realize one child is missing and threaten to shoot another.

Sally and the remaining children barricade themselves inside a cave. One of the older boys recognizes the cave as a place he used to explore and points to an alternate exit. Sally and several of the older students explore the caverns and discover an exit across an underground river.

The group exits the cave and heads toward the countryside, only to discover a farmhouse ahead. The kidnapping crew reaches the farmhouse first; when the old man resists, they shoot him and his wife. Father Christmas objects to the murders and kills another kidnapper.

Sally and the children are locked in the barn with an armed kidnapper while the others go on a supply run. The oldest schoolgirl, Narelle, distracts the kidnapper with flirtatious talk while a boy steals his shotgun. The plan works and the kidnapper is killed, but a stray shotgun blast badly injures the boy who stole the weapon.

With the kidnapper dead, the children and Sally flee into the Outback and resolve to fight back rather than wait for rescue. They abandon passivity and begin to fortify their position, planning to defend themselves against the remaining captors.

They discover a cave suitable for defense and begin fortifying it, drawing on Sally's classroom lessons about Indigenous hunting methods to dig a spike pit and fashion spears from school supplies. The group practices and lays traps, turning their environment into a makeshift fortress.

One of the kidnappers falls to the spike pit, and Father Christmas reaches the cave to confront Sally and the kids. A brutal confrontation follows as they defend themselves against the intruder.

Several months pass before police investigators visit the school to question Sally about Father Christmas's body. Sally maintains she has no knowledge and claims they killed him by rolling a boulder onto him. The investigators are unnerved by the presence of the children, who suddenly surround them and depart without pressing further.

Sally regards the preserved human heart on display among the school's science experiments and seems to derive a strange sense of closure from it. Satisfied, she dismisses the children for the day, signaling a fragile balance has returned to the school.