This is the third book in the Warriors: The New Prophecy series, about the cat clans that live in the forest. In this book, the cats who were chosen to follow the prophecy have returned, but they return to clans that are sick and dying from lack of prey. The humans (called "Two-Legs" here) are encroaching on the forest, destroying all in their path to build new roads ("Thunderpaths"). The clans have no choice but to move, but they must go together, or risk losing their guiding ancestors, Star Clan.
In this part of the story, the main themes are teamwork and friendship, regardless of hardship and differences. The clans have to learn to work together to survive. They must leave the forest together, and travel together to find a new home. On the way, they learn to believe in the strength within themselves, and with each other.