Hugh and his family spend the summer of 1943 on the South Carolina coast. To pass the time, he scans the Atlantic Ocean through his binoculars, looking for signs of enemy activity. One day Hugh sees something in the water that looks like a periscope. Later he plucks a black bag out of the surf. Inside is a crudely drawn map. Then one night he spots a light flashing from the cupola at the top of an abandoned beach house. Now Hugh is convinced that enemy soldiers have invaded the coastline!
Hugh finds clues suggesting that a German submarine is in the vicinity of his family's beach house.