Mice from all over group up and create a plan to drive away the cats, who threaten their livelihood. They created the Giant Mouse of Minsk, which they call their "Secret Weapon," and used it to scare away all of the cats in the city.
When the other mice warn of a cat, Fievel doesn't believe them. His father once told him that cats don't exist in America, and he shares that belief. He changes his mind and runs for his life when a black cat attacks him in an attempt to eat him.