In a quiet wood, a common gray squirrel is going to start a war. It begins when a boy befriends and names him Jack, then teaches him words-and how to use simple machines like the wheel. Jack shares what he's learned with Sister but she's more interested in a meadow squirrel. When construction machines invade the wood, Jack wants to save his tree and nest. He wants stop them. He asks Owl, the old Mother of the forest, to call a Gathering. The animals panic when he tells them he wants to fight the machines to save the wood. He shows the animals how to roll stones and makes a plan. Fox and Rat are suspicious, and do not want to help—until Jack reminds Fox that his den is closest to the machines. Almost all the animals, Fox, Beaver, Rat, Bird, Buck, and even Sister, agree to Jack's plan. Together they work to destroy the machines, but don't finish. They try again—and while they are successful, the humans become very interested in Jack. The animals may win the battle, but not win the war.

After leading the battle against the construction machines to save his woods, Jack the squirrel continues visiting his human friend, Collin. He brings Sister, and Bird, who can copy Collin's words. This allows Jack to 'talk' to Collin. When Jack shares what he learns with the other woodland animals, Fox wants no part of learning anything human. Rat is too busy teasing the humans who spy on the animals. When Rat gets tagged and caged, Jack has to figure out how to save him while avoiding the humans who want most to capture him—and won't stop until they do.

Even though his woods are safe from construction machines, Jack the squirrel can't go back. The scientists who captured Rat, and his sister's mate Jerk, are searching for him-and show up at his human friend Collin's house. To save Jack, Collin and his mother take him to sanctuary. There, Jack meets Addy and other rescued squirrels, along with Horse and a group of barnyard animals. Collin visits, bringing Mina the talking bird, and teaches Jack the last simple machines: the pulley, the lever, and the gear. Jack shares what he learns with Addy, but Hawk and the scientists are hot after him. One final battle between humans and animals erupts. If Jack wins, he'll have to run away from everything he knows If he loses, he will never be free.