In the 2–10 GPa pressure range, ammonia hemihydrate H$_2$⁢O:(NH$_3$)$_2$ (AHH) is a molecular solid in which intermolecular interactions are ruled by distinct types of hydrogen bonds. Upon heating, the low-temperature ordered $P2_1/c$ crystal (AHH-II) transits to a bcc phase (AHH-$p$ ⁢bcc) where each site is randomly occupied by water or ammonia. In addition to the site disorder, experiments suggest that AHH-$p$ bcc is a plastic solid, but the physical origin and mechanisms at play for the rotational and site disordering remain unknown. Using large-scale (∼10$^5$  atoms) and long-time (>10  ns) simulations, we show that, as temperature rises above the transition line, orientational disorder sets in, breaking the strongest hydrogen bonds that provide the largest contribution to the cohesion of the ordered AHH-II phase and enabling the molecules to migrate from a crystal site to a neighboring one. This generates a plastic molecular alloy with site disorder while the solid state is overall maintained until melting at a higher temperature. The case of high (P,T) plastic ammonia hemihydrate can be extended to other water-ammonia alloys where a similar interplay between distinct hydrogen bonds occurs