  The Simple Inclusion Process (SIP) interpolates between two well-known
lattice gas models: the independent random walkers and the
Kipnis-Marchioro-Presutti model. Here we study large deviations of
nonstationary mass transfer in the SIP at long times in one dimension. We
suppose that $N\gg 1$ particles start from a single lattice site at the origin,
and we are interested in the probability $\mathcal{P}(M,N,T)$ of observing $M$
particles, $0\leq M\leq N$, to the right of the origin at a specified time
$T\gg 1$. At large times, the corresponding probability distribution has a
large-deviation behavior, $-\ln \mathcal{P}(M,N,T) \simeq \sqrt{T}
s(M/N,N/\sqrt{T})$. We determine the rate function $s$ exactly by uncovering
and utilizing complete integrability, by the inverse scattering method, of the
underlying equations of the macroscopic fluctuation theory. We also analyze
different asymptotic limits of the rate function $s$.
