For a cold plasma that is inhomogeneous (in the direction across an external homogeneous magnetic field), the nonlinear equation describing the spatial structure and temporal behaviour of a non-stationary disturbance in a resonance layer is obtained. The matching conditions for a disturbance through the resonance layer are obtained, and in the linear limit give a well-known linear matching. It is shown that the spatial and temporal behaviour of the resonance disturbance and the evolution of the resonant absorption in terms of nonlinear theory are determined by the ratio of the nonlinear to linear non-stationary spatial scales. The spatial–temporal profile of the disturbance in the resonance layer and the resonant absorption for different values of this ratio are calculated. A nonlinear decrease in the resonant absorption and a stratification of the resonance disturbance are revealed.