After the events of The Wall of Water, a young mermaid finds herself drifting between two worlds. One world is completely submerged in water while the other is an arid wasteland. In one world, the underwater kingdoms have forged a lasting alliance with the remnants of humanity. In the other, marauders roam a deserted planet in search of water. In one world, submarines and aquatic beings look for whatever remains of civilization. In the other, marauders aim to hunt down the mermaid as they view her as a self-replenishing source of water. Pearl is the only hope of merging the worlds together and restoring a reality in which sea and land peacefully coexist.