Morality
At the center of the novel is a discussion surrounding the theme of morality. At the start of the novel, Ellie and her friends are groups unconcerned with their deaths. In fairness, they had no reason to be. They were young, healthy, and in great shape. They naturally had no reason to think of death or their deaths specifically. But when the unidentified force invades their small and quaint town, they are forced to confront their mortality because of the foe's aggressiveness. Many people in the group began to fall ill and/or ultimately died because of the illness or because the force killed them.
In having to deal with their morality, the group grew as people. They learned from their mistakes, and they got a very harsh and very quick lesson in mortality from the circumstances they were in (a lesson that was unavoidable).
Coming-of-age
Similarly, while coming face-to-face with their morality, the group must quickly mature and come of age. In that sense, Marsden's novel is a coming-of-age novel that tells the story of a group of characters coming head-to-head with a foe much bigger, stronger, and older than them. In fighting (and ultimately beating) the enemies, the group must quickly transition from childhood to adulthood, something each of them is readily able to do.
The importance of friendship
The importance of friendship is one of the biggest themes in Marsden's novel. At the start of the novel, Ellie and her friends go on a camping trip together. They do so to let some steam off and to bond with each other. When things go wrong and the unidentified force invades their town, however, their friendship is put to the test because of the violence around them.
Few friendships must go through what Ellie and her friends went through with the force. Still, they banded together, and their friendships were stronger as a result. Without their friendships, the enemy force would have been able to venture into the rest of Australia and each of the teenagers would likely have been captured by the enemy force.
Death
Death is one of the bleakest, but most important themes of the novel. In the novel, countless people die, including townspeople, teenagers, and presumably members of the occupying force. Each dies for a reason — as nonsensical or reasonable as that reason may be — and the people around them had to deal with the deaths around them. Naturally, this causes internal turmoil within each character in the book.
