Ursula Violet Kronborg might be a princess, but she feels more like a prisoner.

Ever since her parents died in a shipwreck two years ago, Violet has wondered if the rumors are true and her family is cursed. After all, her cousin Tristan never fully recovered from the plague that swept through Elsinore. Her grandfather, King Eric Kronborg, seems to be getting more paranoid by the day. And after the incident on Violet's birthday last year, it's no secret that everyone in Elsinore thinks she's some sort of witch.

Violet's eleventh birthday gets off to a rotten start when she discovers a weird silver streak in her hair. But when a clumsy moment at her party leads to Violet accidentally unleashing powers she didn't know she had, Tristan disappears-and Violet's life is turned upside-down.

As the king's paranoia reaches new heights, Violet accepts the help of a mysterious sea witch named Hydra and sets off on an undersea adventure to rescue Tristan. From a close encounter with a shark to exploring a sunken ship and tricking a crafty group of sea otters, Violet will stop at nothing to save her cousin. But she quickly learns that there's more to Hydra-and her own family-than meets the eye.

Rescuing Tristan means waking up a long-dormant evil in the deepest part of the ocean. Now Violet must learn the full extent of her powers before the most feared sea serpent of all time destroys her family, her home...and herself.