It's March 1942, three months after the Japanese attack on Pearl Harbor to start World War II. Robert Matsui, his wife Rose, and twelve-year-old daughter Ann have moved from Bakersfield, California, to Middleton, Minnesota, during the two-month period when Japanese-Americans are allowed to voluntarily relocate from the west coast.

The Matsuis are refused a promised rental home by a bigoted landlord, but Joe and Fred Beck offer the family a vacant house on the Middleton Ridge near their residence. The reclusive brothers are under surveillance by the FBI as Nazis.

Twelve-year-old Mark Penn is playing war on the ridge when he happens upon the Matsuis, and sees Joe Beck give Mr. Matsui a Luger. Mark tells his pal Swede Larson, and they report their discovery to Police Chief Karl Morton, who is the local FBI liaison. Morton orders them to stay away from the Matsuis. The boys ignore him and soon become so enmeshed in the FBI's intrigue that their lives are endangered.

Mark is the batboy for the Middleton Aces. His best adult friend and former Aces' second baseman Red Collins is fighting the Japanese in the Pacific aboard the USS Yorktown. Mark is too young to join the real battle, but he hopes that he can aid the war effort by finding out who the Matsuis really are. He soon discovers that playing war can be a deadly game.