Nicholas Mettleson is angry when he learns that he and his younger twin sisters will be sent off to spend the summer with their father's uncle in a small lakeside town in Ohio. But, once they get there, it's hard not to like Uncle Nick and his dog, Pistol. The summer seems to promise lots of adventure, from the spectacular view from his bedroom to Uncle Nick giving them sailing lessons on the lake. Nicholas soon meets a friend, Charlie, who can throw a crazy curve ball, and the two of them spend hours sailing together. Then, Nicholas and his sisters find a secret compartment in his room. In it is old film from a movie called The Seaweed Strangler and a love letter from their dad to a woman named Franny. Nicholas teams up with Charlie to solve the mystery and figure out what connection his father has with the lake.