We consider the following Hamiltonian equation on the $L^2$ Hardy space on the circle, $i\partial _tu=\Pi (|u|^2u)\ ,$ where $\Pi $ is the Szegő projector. This equation can be seen as a toy model for totally non dispersive evolution equations. We display a Lax pair structure for this equation. We prove that it admits an infinite sequence of conservation laws in involution, and that it can be approximated by a sequence of finite dimensional completely integrable Hamiltonian systems. We establish several instability phenomena illustrating the degeneracy of this completely integrable structure. We also ify the traveling waves for this system.