Jajamaru, a five-year-old wildcat, lives on Smiling Island, a subtropical place existing beyond conventional time and space. He shares daily adventures and occasional disputes with his friends: Piccolo, a three-and-a-half-year-old Humboldt penguin, and Porori, a four-year-old mouse. His animated form adapts his original puppet design while retaining recognizable features.

Demonstrating adventurous and supportive traits, Jajamaru helps Porori build a boat for a journey to Poroporo Island upon learning of Porori's newborn brother. He actively joins the seafaring voyage, braving a storm that nearly drowns him. Later, his impulsive fishing attempt hooks a sperm whale, which inadvertently carries their boat to an unknown island, stranding the group.

Exploring the unfamiliar island separately, Jajamaru encounters the quarreling pirates Kitan (a fox), Medaruma (a monkey woman), and Goronza (a bear). He helps resolve their conflict and aids in discovering a hidden pirate treasure within a mountain cave—treasure later revealed to belong to Porori's grandfather, Captain Kajiriacci. When an earthquake strikes the cave, Jajamaru assists the group's evacuation, leaving the treasure behind.

Jajamaru shows bravery during crises and loyalty to friends. He exhibits a recurring fondness for bananas. After the island adventure, he reunites with the group as their boat miraculously reappears. He shares in the whale-assisted journey to Poroporo Island and participates in meeting Porori's newborn brother, joining Piccolo in holding the infant.