The kiss isn't fanservice window dressing, it's the entire mechanism of power. Mamori transforms into a weapon the moment Mirei actually commits to her, and the show never pretends that's anything less than romantic. You're watching two girls save each other by loving each other, framed as sci-fi action instead of apology. The ecchi isn't separate from the story; it's the story. Not everyone signs up for that level of commitment to the bit, and the show doesn't care if you don't.
Mamori wakes up on Mermaid, one of five isolated islands where girls with special powers are sent. Within hours of arrival, she's hunted down by other students who see her as an easy target. Just when things look hopeless, Mirei appears and chases them off, but the attackers keep coming back for more. In a moment of desperation, Mamori and Mirei discover something impossible: when they kiss, Mamori transforms into a weapon. Mirei now holds a beautiful sword powered by Mamori's abilities, and together they become something far more dangerous than anyone on the island expected. What started as a survival nightmare turns into something neither girl saw coming. Mamori learns the island has its own rules and its own wars between factions, all while figuring out what she's become. Mirei discovers that protecting Mamori matters more to her than winning battles. As the two navigate the island's chaos and uncover the truth about why they were really sent here, their connection deepens in ways that complicate everything. Action and emotional stakes collide as they face stronger enemies and harder choices.