To twelve-year old James and his younger sister Susie, World War II seems as if it will never end. But the sleepy little town of St. Bees on England's western coast seems an unlikely place for the Nazis to attack, so the children, staying with their Aunt Charlotte, seem safe for now.Against the wishes of his aunt, James joins with a group of other boys to visit an old abandoned coalmine on the outskirts of the village. They're looking for adventure and perhaps even a glimpse of King Arthur's treasure, which local legend claims is somewhere to be found in St. Bees. The fun ends early when Susie shows up to drag James back home to their aunt. But now a real danger presents itself. The children accidentally come across a Nazi raiding party, seeking to destroy an experimental radar station secretly housed in St. Bees. But when things look bleakest, King Arthur and his Knights, fulfilling a vow to return in England's "hour of greatest need," appear to rescue the children.