A theory is presented to describe fluctuation dynamics in magnetized plasmas with impurities. In particular, it is shown that impurities can significantly facilitate an abrupt transient increase of fluctuation amplitude. To demonstrate this, a fluid model is derived to describe how impurities enter fluctuation dynamics. At the linear level, a wave similar to a drift wave can be excited in the presence of impurities. The nonlinear dynamics of this wave is formulated via modulational analysis, and it is demonstrated that drift waves with impurities can develop into a breather, a nonlinear wave that exhibits transient increase of amplitude. Our model indicates that nonlinear breathers become easier to be excited as impurity concentration increases. Breathers transiently increase fluctuation amplitude, and hence may be important to expel impurities. Implications on basic experiments and magnetic fusion are discussed as well.