The fourth installment of the I Marine Project. In a city composed of an aquatic metropolis centered around an orbital elevator and an underwater town inhabited by fish-people, I Marine spends her days diligently making deliveries astride her beloved vehicle, Whalemaru. Together with friends like Wendy, the mayor's daughter, her childhood friend GT who works as a cargo handler, and GT's younger sister Unii, she lives peacefully, loving the sea. However, I Marine occasionally hears a sorrowful singing voice. Today is the city's annual festival, celebrated by all—humans and fish-people alike. Yet, within I Marine, a mournful voice from nowhere grows increasingly louder. Just as the festival is about to end, the voice reaches its peak, triggering a cataclysmic event. At its center stands a boy, singing while weeping in sorrow...