Country high school student Ellie (Caitlin Stasey) sets off on a camping trip with close childhood friend Corrie (Rachel Hurd-Wood), together with Corrie's boyfriend Kevin, Ellie's next-door neighbour Homer, love interest Lee and friends Robyn and Fi. After driving Ellie's parents' Land Rover into the mountains, they hike down into a remote valley known as "Hell".
During their first night camping, Ellie wakes to a sky full of military aircraft. Upon arriving back town, the group finds their homes abandoned, with power, internet and telephone lines down. From the hill overlooking Robyn's house, the group sees that the only lights on in town are at the hospital and the showground. Upon reaching the showground, they find that the citizens of the town being are being detained. Ellie witnesses a man being executed with a shot to the head, and in her horror retreats too quickly, being spotted by a searchlight. They flee but are pursued by soldiers into the backyard of a house. Ellie, using Kevin's singlet, lighter and the fuel tank of a ride-on lawn mower, create an explosion that eliminates the squad.
On return to Corrie's house, they find that Lee and Robyn are missing. Ellie and Corrie witness an Australian Boeing F/A-18E/F Super Hornet get shot down by 3 Chengdu J-10 jets. Whilst the group gathers inside to plan for their return to Hell, an enemy helicopter performs a close examination of the house. Homer shoots at and damages the helicopter, which retreats after dropping flares. The group barely escape with their lives, after a jet destroys the home.
That night, Ellie and Homer sneak back into town and find Robyn in her house. Lee has been wounded, and is being treated by Dr Clements (Colin Friels), the local dentist, who informs them that the invading forces are bringing in their vehicles and equipment from ships moored in Cobbler's Bay over the Wirrawee Bridge. After a brief skirmish with a pair of armed buggies, Robyn, Homer, Lee and Ellie meet back up at Corrie's home. They decide to return to Hell.
On the way they stop at a random house, and are greeted by school mate Chris, who is incredibly stoned and has no idea that a war is going on. Chris joins the group. They returns to Hell, with plans to use it as a secluded hideout where the enemy forces will not find them. While there, they hear a radio transmission revealing that Australia has been invaded by "The Coalition Nations" from nearby Asia, who believe that they have a right to the country's vast natural resources and wealth in order to sustain their growing populations. The transmission also reveals one of the three main ports being used to deposit soldiers into the country is nearby Cobbler's Bay, the only exit from which is the Heron Bridge, which the group then makes plans to destroy.
The group sneaks back to Wirrawee, and devise a plan to blow up Heron Bridge. Ellie and Fi steal a petrol tanker from the council depot. They park it near the bridge and, while waiting for the rest of the team to take their positions. After being discovered by guards, they rush to drive the tanker forward under the bridge. The plan necessarily brought forward, Homer and Lee scare a herd of cattle onto the bridge, forcing the sentry guards to flee their posts, allowing Ellie to park the tanker under the bridge unhindered. Working together, they manage to explode the tanker. Corrie however is shot in the back as the group escapes.
Despite certain capture, Kevin decides to drive Corrie, who is seriously wounded, to the hospital and remain by her side. The group return to Hell. Ellie then finishes her video log, revealing their ongoing guerilla war, the necessity to fight, and that they have not been found.