Nita's family goes on vacation with Kit (and his dog Ponch) to the South Shore of Long Island. One night while Nita is swimming in the ocean, she finds a dolphin in the water. She greets it with The Speech, the magical language of Wizardry. The dolphin excitedly replies "A Wizard!" and swims away. Kit and Nita meet at the shore, and Kit tells of Nita of the rocks on a jetty; something bad is coming to the shores, and the rocks remember in fear.
Kit and Nita go out the next night to search for the Dolphin, much to the suspicion of Nita's parents. They prepare a spell to walk on the water. The dolphin finds them, and tells them that a wizard is being attacked. They are carried by the Dolphin to a nearby beach, where they see a pack of sharks attempting to devour a humpback whale wizard named S'reee, who is also being defended by a group of dolphins. Kit and Nita freeze the sharks, shield S'reee and the dolphins, and release the sharks at S'reee's request. Nita heals her, and Nita and Kit return to the beach house, exhausted, where they discover from their manuals they are on active assignment again.
S'reee and the other whales are short of wizards willing to sing in the Song, and ask Nita and Kit to help. S'reee is in charge of organizing the Song of the Twelve, a gathering of wizards to reenact the time when the Lone Power, the source of all evil, tempted the sea creatures. As the story goes, three whales accepted the Gift, three were undecided, and three rejected it. A Tenth whale, the Silent Lord, was the final vote for all the whales. Instead of accepting, refusing, or not choosing, she killed herself, and was eaten by the Master Shark. This action bound the Lone Power for a time, and succeeding Songs have kept it bound.
Nita volunteers herself to be the Silent Lord, as she doesn't believe she has a good singing voice. S'reee takes Nita and Kit with her to help find other whales for the upcoming Song. Nita, since she has shared blood with S'reee while healing her, turns into a humpback whale without external aids. Kit is given a whalesark by S'reee--a wizardly web which turns him into an enormous sperm whale. After their first day out in the ocean, Kit and Nita return home very late, and Nita's parents don't want them outside anymore.
The next day, Nita and Kit use wizardry to sneak out of the house to help S'reee find more whales for the Song. Kit and Nita meet Ed'Rashekaresket (or Ed), the Pale Slayer, a shark who is to be present for the Song. Nita discovers to her horror that the Silent Lord is actually eaten at the end of the Song - it is not merely a reenactment, but rather a sacrifice ceremony. Since Nita is bound to the job, she cannot pull out, or the whole Song will be sabotaged.
Meanwhile, outside of Long Island, the sea floor is starting to act up, giving signs that the Lone Power's binding is weakening. Large krakens (mutated giant squids) are attacking the wizards underwater, and volcanic vents are increasing the temperature. Nita realizes that she can't back out of the Song now, because this destruction will eventually becomes large enough to destroy the entire Eastern Seaboard.
Nita and Kit return to their home, very late at night. Dairine finds them, gives them their clothes (when they turn into whales, they can't be wearing anything) and allows them time to dry off and prepare an explanation for Nita's mom and dad. Nita and Kit decide that they must tell her parents about wizardry, because when they perform the Song, they will be gone for several days. Kit and Nita levitate and walk on water, but Nita's dad insists on hypnosis. Finally frustrated, Kit and Nita take them to the Moon - so they could see the Earth from above. This finally convinces Nita's parents to accept their wizardly children, and allow them to save the Eastern Seaboard. However, Nita doesn't tell them about her upcoming sacrifice.
Their trouble with the parents now over, Kit and Nita go to bed to prepare for the singing of the Song. Dairine looks at Nita's Book, and discovers she can read parts of it - a sign she also wizardly talent.
Nita and Kit leave the beach house and meet S'reee, who takes them to the practice area for the Song. Nita meets the other whales in the Song, and talks to Ed about death. She realizes that Ed is the only creature in the ocean who is still alive from the first Song of the Twelve. The two become very unlikely friends.
The procession (the ten whales including Nita and S'reee, Kit as security and Ed as the Master) start to descend into Hudson Canyon. They are relentlessly attacked by kraken and other monsters until they reach the site of the Sea's Tooth, the traditional place where the song is held. S'reee talks about how one Song went wrong, and a continent fell into the ocean; Kit and Nita realize she is referring to Atlantis.
The Song goes decently, until one of the whales loses her will and succumbs to the tempting of the Lone Power. She escapes, but returns to allow the Krakens to attack the Singers. S'reee remakes the spell with Kit in the song. Nita goes to perform her sacrifice, but the Lone Power emerges from its binding as an enormous serpent.
A fierce battle ensues. Kit reverts to whale speech, showing that the whalesark is in danger of failing. Ed asks Nita for her wizardry power; she gives it to him, and he finishes her part of the Song. He then attacks the Serpent, who wounds him. The Master Shark's blood in the water calls all the sharks in the area to him; they devour his body, and the Serpent, and the krakens. The Song is completed, and though unusually, it still binds the Lone One.
Nita and Kit return to the beach house, exhausted. They promise to tell the story to Nita's parents as soon as they are better. Nita checks her manual, and discovers that a payment she and Kit owed to the Powers That Be was paid by the Life Service. She and Kit go out into the ocean one last time, and enter Timeheart: the center of the Universe, where no one dies. They get a last glimpse of Ed, before Kit asks for the Powers to bring them their next job. Nita agrees.