Square's Tom Sawyer is based on the 19th-century novel "The Adventures of Tom Sawyer" by Mark Twain, featuring many characters from the book. The plot takes place in 1855 along the Mississippi River in the fictional town of St. Peters, Missouri. The game begins with Tom Sawyer dreaming that pirate treasure is buried somewhere in the southern region. The game uses a traditional RPG format, where characters can level up by gaining experience through battles and completing quests. It also involves using various items and upgrading weapons to help fight enemies with different abilities.