Jim Hawkins (character), the young assistant of the keeper of the ''Admiral Bembo'' inn, and his mouse friend Gran (Rex in English) are one night asked by a Billy Bones for a room and to watch out for suspicious-looking characters. The latter, a band of black-cloaked assassins, soon arrive, and the man asks Jim to take care of the casket he's been carrying before engaging the intruders. Jim and Gran narrowly escape. After they return to the ransacked inn later, they open the casket in hopes of gaining some money as compensation for the damage. Inside they find a map to the treasure hidden by the infamous pirate Captain Flint.