Thirteen years ago Rikki, Cleo and Emma became mermaids! And their lives were never the same. They faced challenge after challenge together, and always pulled through. Now though, their lives are very different. Having gone their separate ways the girls have built their own lives. But the sea is always calling. New dangers lurk. Secrets await discovery. Life as the mermaids know it is about to change forever.

A few years after the events of MXR: Endgame, Francis has created an identical copy of Axol's Inkweaver, complete with unlimited inkling ink. Now, with the entire Mushroom Kingdom at war, Turlandb must enlist the help of a few of his friends to save his universe from Francis.