  We present a NuSTAR and XMM-Newton monitoring campaign in 2014/2015 of the
Compton-thick Seyfert 2 galaxy, NGC 1068. During the August 2014 observation,
we detect with NuSTAR a flux excess above 20 keV ($32\pm6 \%$) with respect to
the December 2012 observation and to a later observation performed in February
2015. We do not detect any spectral variation below 10 keV in the XMM-Newton
data. The transient excess can be explained by a temporary decrease of the
column density of the obscuring material along the line of sight (from N$_{\rm
H}\simeq10^{25}$ cm$^{-2}$ to N$_{\rm H}=6.7\pm1.0\times10^{24}$ cm$^{-2}$),
which allows us for the first time to unveil the direct nuclear radiation of
the buried AGN in NGC 1068 and to infer an intrinsic 2-10 keV luminosity
L$_{\rm X}=7^{+7}_{-4} \times 10^{43}$ erg s$^{-1}$.
