The film begins with a video log by Ellie. She asks the camera how she can tell their story. She suggests to herself "from the beginning."
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, high school crush 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 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 jet fighter being shot down by unidentified aircraft. 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 to Hell, they stop at at a house, which turns out to be schoolmate's Chirs Lang. They are greeted by Chris, who is incredibly stoned and has no idea that a war is going on. He tells the group a story about finding his neighbours shot after the power went out. Chris took first watch, but falls asleep. Meanwhile, Ellie and Lee kiss. To escape having to talk about what happened, ELlie leaves to check on Chris, only to find him asleep, just as an enemy patrol pass by the house. She then threatens to kill him. This scare aside, Chris joins the group. They return 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. Taking out the petrol soked rope from the tanker, Ellie and Fi find they have no matches. However, Robyn and Chris are hiding behind a nearby toilet block in the park. Chris runs out and lights the rope with his lighter. With this, they manage to explode the tanker, which utterly destroys the bridge. 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. After a teary goodbye, the group return to Hell. It then cuts to Ellie's video log, revealing the ongoing guerilla war, the necessity to fight, and that they have yet to be found. The movie ends with the group at the top of the mountain, all in full black and carrying weapons. They look out to Cobbler's Bay, and then decend into Hell.
- Caitlin Stasey as Ellie Linton, the film's protagonist and Lee's romantic interest.
- Rachel Hurd-Wood as Corrie McKenzie, Ellie's best friend and Kevin's romantic interest.
- Lincoln Lewis as Kevin Holmes, Corrie's romantic interest.
- Deniz Akdeniz as Homer Yannos, Ellie's next-door neighbour and Fiona's romantic interest.
- Phoebe Tonkin as Fiona Maxwell, Homer's romantic interest.
- Chris Pang as Lee Takkan, Ellie's romantic interest.
- Ashleigh Cummings as Robyn Mathers,
- Andy Ryan as Chris Lang,
- Colin Friels as Dr. Clement, the Wirrawee dentist
- Don Halbert as Mr. Linton
- Olivia Pigeot as Mrs. Linton
- Stephen Bourke as Police Officer
- Kelly Butler as Mrs. Maxwell
- Julia Yon as Mrs. Takkam
- Dane Carson as Mr. Mathers
- Matthew Dale as Mr. Coles
- Gary Quay as Senior Soldier
- Michael Camilleri as Tanker Driver
- Steer- Missy Higgins (Opening song: seeing Ellie on the tractor on her farm, and Ellie riding to Corrie's on her motorbike)
- Honeymoon Is Over- The Cruel Sea (Introducing Kevin, at the practice cricket pitch)
- Cosmic Egg- Wolfmother (Introducing Homer, walking out of the police station)
- Restaurant Piano- Guy Gross (Introducing Lee, playing piano in their house, and working at his family's restaurant)
- Fader- The Temper Trap (The group driving in the Land Rover up the mountains, heading towards Hell)
- Don't You Think It's Time- Bob Evans (The first night of their camping trip, as the group sit around the campfire. Continues to the next morning, with Lee making a new fire, and Kevin and Corrie make out)
- Black Hearts (On Fire)- Jet (Playing football in the water)
- Flame Trees- Sarah Blasko (Ellie sits in the cubby house while she drinks tea and talks with Corrie)
- Poison In Your Mind- Powderfinger (They drive off again, after Ellie threatens Chris with the gun when he fell asleep on watch)
- Tomorrow- Nic Cester, Davey Lane & Kram (First song of end credits)
- Orchestral Music- (quite frequently throughout, during the explosions, and lead up towards explosions, etc.)