  On the basis of simple principles we derive and investigate the equations of
relativistic plasma magnetohydrodynamics (MHD) in an arbitrary gravitational
field. An exact solution describing the motion of magnetoactive plasma against
the background of the metric of a plane gravitational wave (PGW) with an
arbitrary amplitude is obtained. It is shown that in strong magnetic fields
even a sufficiently small amplitude PGW can create a shock MHD wave,
propagating at a subluminal velocity. Astrophysical consequences of the
anomalous plasma acceleration are considered.
