Natsumi's vacation takes a strange turn when two aliens from the ocean, Meru and Maru, come to visit her. Soon she becomes the Deep Sea Princess, and she grows attached to Meru, who is troubled by a painful past. The Keroro Platoon gets used to her new title and looks forward to a quiet getaway, but the break does not last. A strange alien creature that Fuyuki has taken in reveals itself as an enemy of the kingdom and begins sending destructive monsters to kill Natsumi and anyone who gets in their way. The holiday becomes a fight for survival. The film mixes comedy, action, adventure and drama with the science fiction of the franchise.