  An exact, plane wave solution of the gravitational field equations is
investigated. The source stress tensor is represented by an anisotropic null
fluid with energy flux to which the energy density $\rho$ and all pressures are
finite throughout the spacetime. They depend on a constant length (taken of the
order of the Planck length) and acquire Planck values close to the null surface
$t - z = 0$, the $z$-axis being the direction of propagation. The timelike
geodesics of a test particle are contained in a plane whose normal has constant
direction and the null trajectories are comoving with a plane of fixed
direction.
