Sixth-grader Ben Pratt's life is full of changes that he does not like--his parents' separation and the plan to demolish his seaside school to build an amusement park--but when the school janitor gives him a tarnished coin with some old engravings and then dies, Ben is drawn into an effort to keep the school from being destroyed.

As the new Keepers of the School, sixth-graders Ben and Jill must decipher a handful of clues written as maritime riddles to save their school from demolition by a greedy company.

With a key to the Oakes School in his possession Benjamin, his friend Jill, and a new friend continue to seek ways to protect the school and their town but Lyman, the janitor and a spy, has new tricks up his sleeve.

School protectors Ben, Jill, and Robert leap at a final chance to save their school from being torn down to make room for a seaside amusement park, a demolition that is supported by dastardly janitors Lyman and Wally.