The start of Tomorrow When the War Began introduces readers to the main character, protagonist, and narrator of the novel, Ellie Linton. At the start of the novel, Ellie and her friends (Homer Yannos, Lee Takkam, Kevin Holmes, Corrie Mackenzie, Robyn Mathers, and Fiona Maxwell) are on a weeklong camping trip in the bush of Australia. One night, the group finds a sinkhole in a remote area (that the locals call "Hell") which they decide to camp in. While in the sinkhole, the group notices quite a few military planes flying without any lights on. None of them think anything of the military flights and they return to their home of Wirrawee, Australia.
The group returns to Wirrawee but is perplexed when they discover dead animals and people missing from their beloved town. The group splits up to investigate what happened to their town. Quickly, they discover that their town was taken over by an unidentified force. The force has taken the residents of the town captive so that they could use the town as a beachhead in their invasion.
Eventually, Ellie and her friends are spotted by the invading force, who are unhappy to see them free. The invaders pursue Ellie and her group, but they can escape with the help of an improvised explosive device (IED) that they create.
But after the group escapes, the discover that they are missing two people: Robyn and Lee. They group discovers that Lee has been shot and is hiding out in the center of their town, which also happens to be where the enemies are most concentrated. With the help of heavy machinery, Lee is rescued and nursed back to health. Ellie and her group also discover Chris hiding out in his parents’ house in town.
The group then returns to the sinkhole called "Hell" so that they could regroup. But romantic relationships start to blossom with several of the people in the group, including Ellie and Lee, Homer and Fi, and Kevin and Corrie. But the group's food supply is dwindling, and they are forced to raid nearby farms for additional food stuffs.
Despite the bad circumstances they find themselves in, the group decides to wage a guerilla war against the invaders in hopes of driving them back. To that end, the group concocts a plan to use a stolen oil truck to destroy the bridge that goes out from Wirrawee, allowing the invaders to reach the rest of the country. Their plan, which they initially feared would not succeed, is a rousing success with one caveat: Corrie gets shot.
Ultimately, the group gets a bit smaller and go in separate directions. Their mission, however, remained steadfast: eliminate the enemy force and prevent them from getting into the rest of Australia.
