Manatsu Natsuumi, an energetic girl, moves to Aozora City from Minamino Island to live with her mother, employed at the city's aquarium. Meanwhile, in the Grand Ocean Kingdom, Laura, a mermaid, is tasked to find humans who can open her assigned compact to fight back against the Witch of Procrastination. Following the Mermaid Queen's orders, Laura heads off to Aozora City. Meanwhile, Manatsu arrives at the city, reunites with her mother and goes sightseeing. While taking a break, Laura and Manatsu encounter each other with the mermaid explaining her mission while secretly intending to complete it for her own agenda...