A soluble theory of the post-saturation portion of a beamplasma interaction is developed, concentrating on explaining the results of O'Neil, Winfrey, and Malmberg. Analytic progress is made possible by applying a certain constraint-procedure, characterized by the "rotating-bar" approximation, to a Hamiltonian formulation of the problem. The procedure yields, from the original N-particle Hamiltonian H, and new, reduced Hamiltonian H, which has only two particle-related degrees of freedom, and which maintains the conservation laws of energy and momentum possessed by H. The equations of motion coming from H still describe the selfconsistent interaction of a mode of the plasma with the beam particles, as opposed to previous work, and, because 'or the-great reduction in the number of degrees of freedom, explicit expressions for the nonlinear frequency shift, and growth rate, of the mode can be obtained which are in very good agreement with the simulation results of 0' Neil et al.