Three years have passed, and Emma has settled comfortably into her new life. The basement brownies, the troll-green siren Leucosia, and the prehistoric bird George have faded into memory, mere stories she told her best friend Reggie when they were little. Her battle with the evil witch was nothing more than the product of her overactive childhood imagination. But then it starts to rain.

Before she knows it, Emma is back in Under, this time with sick Reggie in tow. With the help of her friends, both old and new, she travels to the mysterious Yesno jungle and a barren island beneath the setting sun, where the choices she makes threaten to destroy everything she loves. She must overcome terrifying challenges both on the sea and below it, all in an effort to save her best friend. With water, water everywhere, what is a girl to think?