  We report high-resolution 1.3~mm continuum and molecular line observations of
the massive protostar G28.20-0.05 with ALMA. The continuum image reveals a
ring-like structure with 2,000~au radius, similar to morphology seen in
archival 1.3~cm VLA observations. Based on its spectral index and associated
H$30\alpha$ emission, this structure mainly traces ionised gas. However, there
is evidence for $\sim30$~M$_{\odot}$ of dusty gas near the main mm continuum
peak on one side of the ring, as well as in adjacent regions within 3,000~au. A
virial analysis on scales of $\sim$2,000~au from hot core line emission yields
a dynamical mass of $\sim80\:M_\odot$. A strong velocity gradient in the
H$30\alpha$ emission is evidence for a rotating, ionized disk wind, which
drives a larger-scale molecular outflow. An infrared SED analysis indicates a
current protostellar mass of $m_*\sim40\:M_\odot$ forming from a core with
initial mass $M_c\sim300\:M_\odot$ in a clump with mass surface density of
$\Sigma_{\rm cl}\sim 0.8\:{\rm g\:cm}^{-2}$. Thus the SED and other properties
of the system can be understood in the context of core accretion models.
Structure-finding analysis on the larger-scale continuum image indicates
G28.20-0.05 is forming in a relatively isolated environment, with no other
concentrated sources, i.e., protostellar cores, above $\sim 1\:M_\odot$ found
from $\sim$0.1 to 0.4~pc around the source. This implies that a massive star
can form in relative isolation and the dearth of other protostellar companions
within the $\sim1$~pc environs is a strong constraint on massive star formation
theories that predict the presence of a surrounding protocluster.
