A quasi-linear perturbation expansion is used to study the evolution of a localized disturbance introduced in a homogeneous plasma characterized by a distribution function having a small "bump" on its tail. For this problem first-order theory of the Vlasov equation predicts the development of growing waves. The curtailment of this instability through a plateau development in v space is given through the second-order corrections. During the curtailment process the distribution function develops weak time and space dependence. There is shown an energy transfer balance between the growing waves and the particles with bump velocity which are part of the "background" distribution function and those which are second order in the "disturbance" distribution. Comparison with earlier quasi-linear theory is made.