A summer at the seashore with Cousin Dorothy is an exciting enough prospect for the Bobbsey Twins, who, of course, don't know of the other adventures awaiting them there.

Visiting the Lakeside Amusement Park, Bert and Nan, Freddie and Flossie, and Dorothy make a new friend, Cindy, whose father has been lost in an airplane crash at sea. And sharp-eyed Freddie catches a glimpse of the thief who has eluded police after stealing a sackful of $100 bills.

Together the children enjoy all the pleasures of the seashore, but strange and intriguing things keep happening too. Nan finds a bottle washed ashore by the waves, containing a message from someone marooned on an island. And there is the mystery of the vanishing boats. First, the canoe is stolen from the water carnival, then Freddie's toy sailboat floats away to seas, and finally the motorboat vanishes from its mooring.

But the Bobbsey Twins, who love mysteries, eventually put all the clues together to find the answer and win a surprisingly reward!