It is the year 1347 AD. A disaster is about to strike. The Black Death is approaching, and during the next few years, large parts of the population of Europe and Northern Africa will be killed by the plague.

In Rattus, the players settle in the various regions of Europe and Northern Africa, while the plague spreads through all these regions. The players gain help from the various professions of the middle ages. Some of these, like Peasants and Bakers, help the players grow their populations. Some, like the Monks and Nuns, use wisdom and faith to avoid the plague, while the warfare conducted by the Knights and Soldiers spreads the plague to new areas. However, the plague does not make any distinction. When the rats arrive, no one can feel safe. Finally, the plague withdraws and the game ends. Only then it will be clear who was able to keep his population alive, and win the game!