Monster Strike: Mermaid Rhapsody

Ren and his friends travel to Okinawa during summer break after Minami's father, a former mercenary, is invited to an American naval gathering. On the beach, Ren encounters a mysterious girl who leaves him with only a desperate plea for help before vanishing.

Unable to ignore her words, Ren begins searching for the stranger and the reason she singled him out. What first seemed like a carefree trip soon draws the group into a hidden danger, and uncovering the girl's identity becomes inseparable from saving her.