Welcome to the Island of Bubbles

Mitten, an island where time flows quietly. Lupin and his friends are trapped on the island due to a bridge failure, and they have an unexpected holiday. A girl, Muru, who works at a café, is curious about the look of the Lupin's crew who appreciate with their free time. With her sketchbook in her hand, the girl began to have delusions of Lupin's relationship. Meanwhile, Fujiko hears the legend of a mermaid that has been handed down from ancient times on the island...?