It's the weekend — a perfect day for a sail off the coast of Maine. But there's something fishy going on with the local dolphins and Abbi, Lydia, and Daniel are determined to find out why. Are the dolphins twisting, leaping, and whistling because they're playing? Or is it a warning of nearby danger? And if it's danger, how can the kids get grownups to believe them? Abbi and her friends are diving head first into this mystery!