Meru, the prince of the Deep Sea, is destined to marry Natsumi Hinata, whom he considers his princess. Having no memories of his childhood, family, home, or race, Meru grows up with a profound sense of loneliness, alongside his servant Maru, who shares in this second childhood filled with isolation. One night, Meru and Natsumi wield the kingdom's sacred weapon, the Mer Ball, which resembles the Kero Ball, to uphold their titles.
Kululu then reveals that Meru and Maru originate from Maron, a water planet, and that Meru intends to flood Pekopon to make it his own world. However, they are saved by the Keroro Platoon, along with Fuyuki and Momoka. With the corrupted Mer Ball now out of dangerous hands, Meru and Maru ultimately fall in love. They bid farewell to their friends as they embark on a new life on Maron as prince and princess.
