Ellie Linton goes out camping in the bush for a week with her friends Homer Yannos, Lee Takkam, Kevin Holmes, Corrie Mackenzie, Robyn Mathers, and Fiona Maxwell. They find a way into a large, vegetated sinkhole in a remote area of bush the locals have dubbed "Hell", and camp there. During this time they see large numbers of planes flying through the night without lights, and though it is mentioned in conversation the following morning, they think little of it, dismissing it as military planes heading back from a demonstration.
When they return to their home town of Wirrawee, they find that all the people are missing and their pets and livestock are dead or dying. Fearing the worst, they break into three groups to investigate Wirrawee's situation. They discover that Wirrawee was captured as a beachhead for an invasion of Australia by an unidentified force; local citizens are being held captive by the occupiers. Ellie's group is spotted, and pursued by the enemy and, in order to escape, use the fuel tank of a ride-on lawnmower to create an improvised explosive. However, after reuniting with Homer and Fi at a pre-arranged meeting point, they discover Robyn and Lee missing. Homer and Ellie search for them and they are met by Robyn.
They discover that Lee has been shot in the leg and is hiding out in the main street of Wirrawee, the centre of the enemy's activity. Ellie and Homer confer with the others and Ellie decides that they should attempt to rescue Lee, using a front-end loader to move and protect him. After a protracted chase that sees several soldiers killed, Lee is successfully rescued and returned to the safety of Hell but not before they discover Chris Lang hiding out in his house after his parents were away on a business trip.
While hiding out in Hell, a romantic relationship forms between Ellie and Lee, Homer falls in love with Fi, while Kevin and Corrie continue a romantic relationship started a few months before the invasion. The teens decide to raid nearby farmhouses, searching for food and other supplies, and then retreat to Hell to establish a base camp for themselves. The group eventually moves toward waging a guerrilla war against the invaders. Ellie, Fi, Lee, and Homer steal a petrol tanker and use it to blow up the main bridge out of Wirrawee. While the raid is occurring, Corrie is shot while she and Kevin are gathering supplies. Kevin takes her to the occupied town hospital, and turns himself in, in exchange for medical assistance.