It's 1599, the Age of Discovery in Europe. Bren Owen, growing up in the small town of Map on the coast of Britannia, is inspired by the arcane maps his father creates as a cartographer for a map mogul named Rand McNally. When a dying sailor gives Bren a strange gift, cracking the hidden code could lead Bren to a fabled lost treasure-- as well as great danger and an unusual friend named Mouse.