Keroro Gunsou Movie 2: Shinkai no Princess de Arimasu!

During a seaside vacation, Natsumi Hinata encounters two mysterious aliens, Meru and Maru, who emerge from the depths and take a special interest in her. Meru chooses Natsumi to become his Deep Sea Princess, drawing her into an underwater realm shaped around a lonely dream of companionship.

Keroro and the platoon refuse to leave Natsumi behind, but rescuing her becomes more dangerous as the strange power beneath the sea grows unstable. What begins with Meru's desire for a princess threatens to remake far more than his own world, forcing humans, Keronians, and the visitors from the deep to confront what Natsumi truly wants.