It's time for another important summer ritual, the family trip to Doinaka Beach, but Hinata Aki is uable to accompany the children due to work commitments, leaving Natsumi and Fuyuki without an adult guardian, until Kururu reveals his latest invention, an age manipulation beam that turns Natsumi into a full-grown adult. An unexpected side effect causes Natsumi to utter 'old-man' jokes involuntarily, while Keroro's hidden angle and true reason for infiltrating the beach is revealed—the super-rare 'Dangale' knockoff model kits, offered as a prize for entering the Swimsuit Beauty Stand-Up Comic Competition.
Natsumi, gracias a la pistola "Solo se vive dos veces" de Kururu se vuelve cuerpo adulto con sus cosas de mujer y acento macarra. Más tarde van a la playa y participan en un concurso de contar chistes en bikini.
Keroro軍曹他們很想去海邊玩，可是媽媽說不能讓小孩子跟外星人單獨到海邊去玩，而阻止了他們。這時候，Kururu出現，用他的發明作品"促進成長槍"把夏美變成了大人，於是第二天，他們一人行一大早就開開心心的到了海邊，但是…
