Peter Stokes writes a paper for his fifth-grade English class about something that really happened: his war with his grandpa.
Peter’s younger sister, Jenny, is the first to tell Peter that Grandpa Jack is coming to live with the Stokes family and that he will be living in Peter’s room. Grandpa Jack lives in Fort Lauderdale, Florida, but after their grandmother died, he grew lonely. His leg hurts him pretty bad, too. That’s why Peter has to move to the third floor, instead of Grandpa living there.
Peter has lived in the same room since he was a baby. Before Grandpa Jack comes, Peter and his parents move all his stuff to the guest room on the third floor. The hallway feels spooky to Peter, and he is a little scared to sleep there by himself. Once he turns out the lights, he hears different noises than he heard in his room.
Then Peter began to grow angry. He doesn’t feel it is fair that he is the only one who is inconvenienced because of Grandpa Jack is staying with them. Peter chooses to fight against this injustice, but he isn’t sure how.
When Dad brings Grandpa Jack home, Jenny gives Grandpa Jack a big hug. Grandpa Jack gives Peter a hug and comments on how he’s grown. Peter does love his Grandpa Jack.
Grandpa Jack doesn’t go anywhere with the family, such as to movies or restaurants. He sleeps a lot or else sits in the living room watching TV or on the porch to watch the neighborhood. Peter’s friends Billy and Steve agree that Peter’s grandpa is a room robber. They tell him to fight for what is his.
Peter types a note that says he declares war on his grandpa because his grandpa has stolen his room. He signs it “The Secret Warrior,” but also writes that Grandpa Jack should not tell Peter’s parents about the note because the war is between them.
Peter waits for a response, but doesn’t get one. Grandpa Jack continues to treat Peter well and even says he likes to be around Peter. This frustrates Peter because he doesn’t know if he can be in a war if he is the only one fighting.
Peter’s friends convince him to attack and not wait for a response. That night, Peter leaves another note from “The Secret Warrior” in his grandpa’s room next to the alarm clock. The note says that people who steal other people’s rooms don’t have the right to sleep well. Grandpa Jack’s alarm goes off at 3 a.m.
Peter hears Grandpa Jack turn off the alarm and come to Peter’s room. They talk about the war. Peter insists that Grandpa Jack has stolen his room, and Grandpa Jack insists that Peter’s parents have given it to him. As Grandpa Jack leaves, Peter lets him know that he loves him, but they are in a war.
The next day, Grandpa Jack wakes up late, but he doesn’t mention anything about the previous night’s escapades to Peter’s mother. He also doesn’t acknowledge Peter at the breakfast table. Later, they meet under a flag of truce. Grandpa Jack talks about how you don’t always get what you want in life. He thinks it might be a good thing that Peter won’t get what he wants, and the flag of truce is over.
Peter then takes his grandpa’s slippers and leaves a note, but Grandpa Jack goes to Peter’s room and finds them. Grandpa Jack tells him to stop with the tricks, but Peter thinks of what he’s doing as guerrilla warfare. That makes Grandpa Jack laugh. He kisses his grandson good night. That’s when Peter is afraid he will lose the war because his grandpa is way too nice.
The next day Grandpa Jack and Peter go for a walk. Grandpa Jack tells Peter that they have a misunderstanding, not a war. He says that the only time someone should go to war is when someone is attacked. Peter says that he was attacked. His territory, which is his room, was taken from him, and he is trying to get it back. Grandpa Jack slaps him, and Peter is stunned. Grandpa Jack tells Peter that war is painful. This makes Peter mad. He leaves his grandpa and hurries home, determined to win this war.
At dinner, Peter isn’t sure he likes being at war with Grandpa Jack. His grandpa smiles and laughs and talks to Peter, but Peter can no longer tell if his grandpa means what he says. That night, Grandpa Jack and Dad play dominoes, and they all applaud when Jenny gives an impromptu ballet.
Another day, Peter and his friends decide to play Monopoly. They find that Grandpa Jack has removed all the game pieces and has left a note signed “The Old Man.” The boys get really upset with each other. That’s when Peter knows Grandpa Jack’s first attack has worked. He has caused three good friends to become really annoyed with each other.
Peter goes to talk with Grandpa Jack, but the man is too smart. He stays with others in the family so Peter can’t say anything to him. The next time they talk is after school while Grandpa Jack fixes a rocking chair in Peter’s room. Grandpa Jack won’t give back the game pieces. He is holding them as prisoners of war. He will give them back when the war is over. He finishes fixing the rocking chair, and Peter kisses him on his forehead to thank him.
Grandpa Jack begins fixing other things in the house. Peter tries to steal Grandpa’s toolbox, but it is too heavy. Then he and Grandpa Jack have an amazing day in the harbor as Grandpa teaches Peter how to fish. That night, Peter steals Grandpa Jack’s wristwatch.
This time Peter hides the watch well and won’t give it back. Grandpa decides to play hardball with his grandson. As Peter waits for his grandpa to retaliate, he faces psychological warfare and becomes anxious.
Jenny brings Peter’s Monopoly game downstairs to play with the family. She sees the note and wants to know if Grandpa Jack has played a trick on Peter. To cover, Peter hurries and gets Grandpa Jack’s watch, and Grandpa Jack gets the Monopoly pieces. They trade so Jenny doesn’t figure out what is happening.
Then Grandpa attacks. He turns off Peter’s alarm clock and hides all of Peter’s clothing and schoolbooks. He even makes him replace the laces in his shoes. Peter barely makes it to class in time for roll call, and he forgets his lunch.
Peter waits to spring his next attack and hopes Grandpa Jack is worrying about when the attack will take place. Then one night Peter steals his grandpa’s dentures. The next morning when his grandpa comes to ask for his teeth, Peter demands his room.
He sees the sadness in his grandpa’s eyes and how old he has become. Peter realizes how much he loves Grandpa Jack. Peter hurries to get the teeth and declares the war over. He apologizes and feels ashamed for what he has done.
They talk about how wars keep escalating, like their war had, and how Peter’s family should have had a family conference, which included Grandpa Jack, to discuss the room situation.
Grandpa Jack understands that Peter was hurt when his family kicked him out of his own room. He comes up with a plan. He works on the basement and makes a little apartment for himself so Peter can have his room back.