Electrons in graphene behave like relativistic Dirac particles which can reduce velocity of light by two orders of magnitude in the form of plasmon-polaritons. Here we show how these properties lead to a peculiar nonlinear plasmon response in the quasiclassical regime of terahertz frequencies... On one hand we show how interband plasmon damping is suppressed by the relativistic Klein tunneling effect. On the other hand we demonstrate huge enhancement of the nonlinear intraband response when plasmon velocity approaches the resonance with the electron Fermi velocity. This extreme sensitivity on the plasmon intensity could be used for new terahertz technologies. (read more)