Porori, a four-year-old mouse boy, lives on Smiling Island, a subtropical realm existing outside normal time and space. He befriends Jajamaru, a five-year-old wildcat, and Piccolo, a three-and-a-half-year-old Humboldt penguin girl. The trio often play and squabble during their shared adventures.

Porori's mother communicates through letters, and his grandfather is the former pirate Captain Kajiriacci. News of his baby brother's birth reaches Porori via letter while he rests outdoors, sparking his immediate journey.

Eager to meet his newborn sibling, Porori resolves to travel to his family's home on Poroporo Island. He takes charge, convincing Jajamaru and Piccolo to help build a boat for the voyage. His grandfather's pirate past fuels his imagination; he adopts playful pirate antics during the trip, accompanied by Pepera, Captain Kajiriacci's singing parrot.

Their sea voyage tests Porori's resilience. A violent storm scatters the travelers, and they accidentally hook a sperm whale that drags their boat off course. These events strand them on an unfamiliar island ruled by three warring pirates: Kitan (a fox), Medaruma (a monkey woman), and Goronza (a bear). Porori explores the island and helps resolve the pirates' conflict.

Porori's pirate heritage becomes crucial when the group discovers a hidden treasure cave containing his grandfather's relics, coinciding with Pepera's return. Though seismic activity forces them to abandon the physical treasure, the experience deepens Porori's connection to his family history. The journey concludes successfully when the sperm whale returns them to Poroporo Island, reuniting Porori with his mother and introducing him to his baby brother.

Throughout this adventure, Porori experiences emotional growth, shifting his focus from self-centered excitement about becoming an older brother to actively caring for his companions during hardships. His interactions with Jajamaru and Piccolo throughout the ordeal further strengthen their friendship.