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.

Les Planétaires sont envoyés dans un univers irréel et violent, dirigé par Skumm et le docteur Blight...