Vacationing on the South Shore of Long Island over the summer, Kit and Nita come to the aid of a wounded whale off Fire Island... and abruptly become two wizards in deep trouble.
The whale is a wizard named S'reee, and she has problems of her own that make nearly being eaten by sharks look minor. The undersea wizards are about to perform an ancient and potentially dangerous ceremony called the "Song of the Twelve" – a wizardry that keeps land and sea apart, and serves other, more secret purposes.
The "Song" must be performed soon, but the whales are a couple of wizards short of the needed Twelve, and S'reee asks Nita and Kit to help.
They agree, and shortly find themselves involved in a complex and increasingly difficult game – turning into whales, to help the undersea wizards complete the the Song, and (at the same time) trying to keep Nita's folks from finding out what's going on right under their noses. But events seem to conspire against them as Nita's bratty little sister Dairine keeps snooping around to find out what Nita and Kit are doing. And Nita discovers, to her dismay, that the role of the "Silent Lord" in the Song of the Twelve is going to pose challenges she never expected... deadly ones.
The ancient ritual goes forward in the dark depths of Hudson Canyon off New York City. But the greatest danger to Nita and Kit turns out not to be the "Song of the Twelve" itself, but its oldest, most mysterious celebrant – the gigantic Master-Shark, ed'Rashtekaresket, who only understands one thing: blood in the water...