Venice's population is relocating due to its pollution and smog affecting the land and structures - but also because of a large mutated group of rats whose bites sicken their victims. The Planeteers arrive and meet a pathologist, Resa, attempting to find a cure for the disease they cause that her young relative Pietre is suffering, and learn through clues that Verminous Skumm is behind it. But it isn't until Ma-Ti is bitten, and badly suffers, that they learn that Skumm is doing it through a substance placed in processed cheese that causes the rats and whoever they attack to become infected by contaminated canal-water. Now they have to find a cure for the worsening patients and stop the rats from terrorizing the populace with the contamination they thrive on.
