From the back of the book:
Live what you believe in... die fighting for it.
The third day comes a frost... a killing frost.
The enemy spreads across the land, cold and relentless. They invade. They destroy. They kill.
Only the heroism of Ellie and her friends can stop them.
When hot courage meets icy death, who will win through?
The Third Day, The Frost is the third volume in the award-winningTomorrow series.
Ellie and her friends, are a small band of teenagers trying to survive in the Australian countryside, and continue to resist the enemies who have invaded their country. In the first novel "Tomorrow: when the war began", the simple idea of going on a camping trip into the bush, ends up saving the group from an oncoming invasion. Ellie and the group continue on in during the time of war in this third novel.
At the start of "The Third Day, the Frost" the characters are emotionally devastated by what they saw and did in "The Dead of the Night" but this is nothing compared to what happens to them by the end of this book. By the end of "The Third Day, The Frost" the survivors are physically safe but crushed - emotionally, physically and mentally - by the experiences they have undergone in these few hundred pages. Yet before that they find the strength within themselves to accept the events of "The Dead of the Night" and again act. This time picking a target with such precision and moving against it with such cunning that they change the course of the war and elevate themselves to public enemy number one. (Sourced from http://www.rsimpson.id.au/books/tomorrow/books/third.html)