Using Galactic Plane surveys, we have selected a massive (1200 M ⊙), cold (14 K) 3.6？70 μm dark IRDC, G331.372-00.116. This infrared dark cloud (IRDC) has the potential to form high-mass stars, and given the absence of current star formation signatures, it seems to represent the earliest stages of high-mass star formation. We have mapped the whole IRDC with the Atacama Large Millimeter/submillimeter Array (ALMA) at 1.1 and 1.3 mm in dust continuum and line emission. The dust continuum reveals 22 cores distributed across the IRDC. In this work, we analyze the physical properties of the most massive core, ALMA1, which has no molecular outflows detected in the CO (2？1), SiO (5？4), and H2CO (3？2) lines. This core is relatively massive (M = 17.6 M ⊙), subvirialized (virial parameter α vir = M vir/M = 0.14), and is barely affected by turbulence (transonic Mach number of 1.2). Using the HCO+ (3？2) line, we find the first detection of infall signatures in a relatively massive, prestellar core (ALMA1) with the potential to form a high-mass star. We estimate an infall speed of 1.54 km s？1 and a high accretion rate of 1.96 × 10？3 M ⊙ yr？1. ALMA1 is rapidly collapsing, out of virial equilibrium, which is more consistent with competitive accretion scenarios rather than the turbulent core accretion model. On the other hand, ALMA1 has a mass ~6 times larger than the clumps Jeans mass, as it is in an intermediate mass regime (M J = 2.7 < M lesssim 30 M ⊙), contrary to what both the competitive accretion and turbulent core accretion theories predict.