Kotan, son of a descedant of Ainu, becomes a friend with a tiger named Dan. The tiger, which escaped from a train when he was sent to a zoo, was rescued by Kotan and his father. However, right after the family saved Dan, Kotan's parents were killed and Kotan himself was raised by an old Ainu who inherited a secret key that shows where the old treasure of Ainu is hidden. The old man was killed by a mysterious group which also is looking for the hidden treasure. Kotan inherited the key from the old man and begins to search for other two keys to find the hidden treasure with Dan.