  Using Galactic Plane surveys, we have selected a massive (1200 M$_\odot$),
cold (14 K) 3.6-70 $\mu$m dark IRDC G331.372-00.116. This 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
H$_2$CO (3-2) lines. This core is relatively massive ($M$ = 17.6 M$_\odot$),
subvirialized (virial parameter $\alpha_{vir}=M_{vir}/M=0.14$), 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
$\times$ 10$^{-3}$ M$_\odot$ yr$^{-1}$. ALMA1 is rapidly collapsing, out of
virial equilibrium, more consistent with competitive accretion scenarios rather
than the turbulent core accretion model. On the other hand, ALMA1 has a mass
$\sim$6 times larger than the clumps Jeans mass, being in an intermediate mass
regime ($M_{J}=2.7<M\lesssim$ 30 M$_\odot$), contrary to what both the
competitive accretion and turbulent core accretion theories predict.
