Chonko the Mouse serves as a forest-dwelling companion animal and Moku the Bear's explicit friend, both capable of conversing with humans. They enter the narrative when Moku suffers an injury, immediately establishing their close bond. Living near the main human family's estate, Chonko and Moku befriend the children Anju and Zushio. The mouse demonstrates loyalty by actively aiding the family against the antagonist Governor Onikura. Chonko coordinates with other forest animals to harass Onikura and his attendants, causing them to tumble into a river.

After the family patriarch's wrongful exile, Chonko accompanies Anju, Zushio, their mother, and their maid into the woods during their flight. The mouse continues collaborating with animal companions to thwart pursuers, gnawing through ropes supporting a bridge after the family crosses, sending the pursuers into the raging river below.

Chonko and the other anthropomorphized animals provide levity and physical comedy, contrasting the somber human drama involving slavery and familial separation. Their exaggerated, cartoonish interventions against human antagonists create this tonal juxtaposition.

During the escape, slave traders drug Chonko and other animal companions, incapacitating them and enabling the human characters' capture and sale into slavery.

Chonko survives these events, appearing in the narrative's conclusion alongside Moku and numerous forest creatures. They are present as the grown Zushio and Saburo pay respects at a wooden statue carved in Anju's image. The character remains consistent throughout, functioning primarily as part of a supportive animal ensemble aiding the protagonists against adversaries.