As WWII engulfs Europe, Nazi saboteurs prepare to strike at America's heartland from beneath the waters of the Great Lakes. Their weapon - a U-boat - has been smuggled piece-by-piece, reassembled, and abandoned in a secret underground factory.

Eight decades later, 16-year-old Charlie LeClair discovers the Last U-boat-deep beneath his backyard on the shores of Lake Erie. He and his friends explore the perfectly preserved boat and the adventure begins.

Can Charlie and his friends bring the sub to life? Can they solve the mystery of how it got there? Can he do it without getting caught?

The answer depends on whether his parents, his friends, a budding romance, the US Navy, an army of terrorists, and a pair of criminal thugs can stop him.