Gran, an anthropomorphic mouse distinguished by round spectacles, serves as a steadfast companion to a human protagonist. Initially aiding in the operation of a rural inn, their routine shifts when a cryptic visitor delivers a treasure map, sparking a perilous quest. After escaping a violent ambush, the pair flees aboard a steam-powered vessel, unwittingly joined by an infant stowaway. Gran’s chatty demeanor inadvertently exposes the map’s significance to pursuing foes, resulting in its theft. Captured by marauding pirates and forced into servitude, the clever mouse devises escape plans, blending strategic sabotage with lighthearted pranks to undermine their captors.

Throughout clashes between rival pirate crews, Gran proves vital in reclaiming the map and navigating tense alliances, including one with a determined young woman connected to the treasure’s hidden history. The mouse’s ingenuity culminates in assisting the activation of an ancient mechanism, draining a volcanic lake to unveil the trove’s resting place. Whether mediating disputes, outwitting adversaries, or fueling chaotic escapades, Gran’s loyalty and resourcefulness remain central to the group’s race to secure the treasure against relentless opposition.