The dynamic plastic response of a submerged free-free beam to an underwater explosion bubble is studied in this paper. A fluid-structure analysis is given to find the fluid force acting on the beam. Assuming that the beam is made from rigid, perfectly plastic material, we obtain the hydroplastic equation governing the dynamic plastic deflection of such a beam. The equation is analytically solved. Examples are given to discuss the features of the dynamic plastic response. It is observed that longer beams sustain larger plastic deformations than shorter beams, but shorter beams undergo larger rigid body motion than longer beams. Most of energy is consumed on plastic deformation rather than on rigid body motion. It is impossible, however, for more than three hinges to form on the beam.