Gran, a mouse, serves as Jim's loyal companion throughout their shared adventure. Together, they operate the Admiral Bembo inn until the arrival of Billy Bones, a one-legged cat pursued by assassins, disrupts their routine. After a confrontation damages the inn, Gran escapes with Jim and discovers the treasure map hidden in Bones' casket, prompting their quest for Captain Flint's fortune.

Accompanying Jim aboard their steam-powered barrel boat, the Pioneer, Gran inadvertently becomes caretaker to the stowaway infant Baboo. Their voyage is intercepted by Captain Silver's pirate crew, leading to capture. Sold into slavery on Pirate Island, Gran is confined with Jim and encounters Kathy, Captain Flint's granddaughter. Gran's unchecked talkativeness reveals the treasure map's existence to the fox pirate Baron, enabling its theft.

Escaping captivity, Gran participates in the chaotic retrieval of the map during the pirate captains' council. Aboard Silver's ship, the Pork Saute, Gran remains beside Jim, assisting in countering pirate schemes—including serving them tempura-fried inedible objects.

Surviving the storm that destroys the Pork Saute, Gran reaches the island with Jim, Kathy, Baboo, and the reformed walrus pirate Otto. Gran witnesses the final confrontation between Jim and Silver, the draining of the volcanic crater lake revealing Flint's sunken ship, and the treasure discovery. Gran departs aboard a commandeered pirate ship with Jim, Kathy, Baboo, Otto, and now-loyal former pirates, leaving Silver and his monkey helmsman Spider adrift.