In 133 AD: Thousands of Roman legionaries guard the border to Germania in the province of Rhaetia. Suddenly, something sinister happens at Fort Vetoniana: more and more soldiers are stricken by a mysterious disease. Have they been poisoned? Germanic merchants who live peacefully on the other side of the Limes are targeted. The Roman boy Magnus and his friend Finn from Germania do not want to believe this and investigate on their own. They sneak onto the estate of a rich landowner and eavesdrop on "Operation Satan's mushroom". A dangerous race against time begins.