In the coastal fishing village of Whispering Wave, where summer brings throngs of tourists and fairy tale entities, a dark terror lurks. As the sun sets, the serene waves become treacherous, releasing the funa-yurei, sinister Sea Phantoms that drown anyone who dares to wander the streets at night. Lorelai Blackwater, the village witch, reveals a grim solution: to quell the nightly horrors, the sea monster umi-bozu demands a virgin sacrifice.

When Yukina, the fiancée of the mayor’s son, is chosen for this fate, Marius Tidecaller refuses to surrender her. He summons the famed adventurers known as the Cursebreakers. This eclectic group includes Henry, son of the Blue Fairy; the reincarnated Frog Prince, Gamaken Kuronuma; the Swan Princess, Odette; the resourceful maid, Meddie; the bunny priestess, Usagi; and the fierce dragoness, Amber. Their audacious plan? Henry will disguise himself as Yukina and offer himself to umi-bozu. But what seems a straightforward mission takes a perilous turn when the Demon Sea King Seth intervenes.

Seth, the Sea Sorcerer, reveals he seeks not a sacrifice, but a bride. A fierce battle ensues, but the Cursebreakers are overpowered. In a desperate bid to save her friends, Usagi offers herself to Seth. As they are taken away, Henry awakens and follows them to Seth’s hidden lair. He challenges Seth to a duel with high stakes: freedom if he wins, servitude as Seth’s bride if he loses.

Despite his valiant efforts, Henry loses the duel. Usagi is sent back to safety by umi-bozu, but Henry is transformed into a merman by Seth's ancient magic. Taken to the underwater Kingdom, Henry faces a dire fate as the Queen of the Sea. Will the Cursebreakers rescue Henry from the clutches of the malevolent Sea Sorcerer, or will he be lost to the depths forever?