  Recently it was shown that the exact cosmological solutions known as the
vacuum plane-wave solutions are late-time attractors for an open set of the
spatially homogeneous Bianchi universes containing a non-inflationary
$\gamma$-law perfect fluid. In this paper we study inhomogeneous perturbations
of these plane-wave spacetimes. By using expansion-normalised scale-invariant
variables we show that these solutions are unstable to generic inhomogeneous
perturbations. The crucial observation for establishing this result is a
divergence of the expansion-normalised frame variables which ultimately leads
to unstable modes.
