During a vacation Natsumi is visited by two male and female aliens named Meru and Maru. After becoming the Deep Sea Princess, Natsumi gains feelings for Meru who is haunted by his traumatic past. Having grown accustomed to Natsumi's title, the Keroro Platoon expects a calm vacation; however, their vacation is cut short and turns into a fight for survival as an alien creature that Fuyuki has adopted reveals itself to be the enemy of the kingdom and now sends creatures of mass destruction to assassinate Natsumi, and any who stand in their way.
심해에 살고 있는 정체를 알 수 없는 존재들에게 납치당한 한별이(나츠미)를 구출하기 위해 케로로와 친구들이 힘을 합쳐 펼치는 코믹 어드벤쳐.
從Ma隆星漂流到地球，長久以來相依為命的Ma隆星後裔Meru王子及隨從Maru兩人，心裡一直惦念著要找到地球上最堅強且溫柔的公主，進而重建自己的王國是也。
然而，Meru及Maru兩人心目中最理想的公主候選人，竟然是KERORO小隊隊長KERORO軍曹所寄住的日向家的長女－－「夏美」是也！KERORO小隊為了解救被綁架到深海神秘王國的夏美，展開了拯救夏美大作戰！
被當作公主的夏美，在探訪根據她的記憶投影而創建的夏美城與城下町的同時，不知不覺中竟開始醉心享受起這種被重視的皇室生活是也。
在此同時，Meru他們也密謀著要將地球變成像Ma隆星一樣—100%的海洋星球是也。
這次KERORO小隊竟然要從侵掠地球變成拯救地球是也，這群傻瓜青蛙要如何救出夏美？又要如何成功地拯救地球？
