We present initial results from ongoing lattice investigations into the thermal phase structure of the Berenstein--Maldacena--Nastase deformation of maximally supersymmetric Yang--Mills quantum mechanics. The phase diagram of the theory depends on both the temperature $T$ and the deformation parameter $\mu$, through the dimensionless ratios $T / \mu$ and $g \equiv \lambda / \mu^3$ with $\lambda$ the 't Hooft coupling. Considering couplings $g$ that span three orders of magnitude, we reproduce the weak-coupling perturbative prediction for the deconfinement $T / \mu$ and approach recent large-$N$ dual supergravity analyses in the strong-coupling limit. We are carrying out calculations with lattice sizes up to $N_{\tau} = 24$ and numbers of colors up to $N = 16$, to allow initial checks of the large-$N$ continuum limit.