  We present a K-band spectroscopic study of the stellar and gas kinematics,
mass distribution, and stellar populations of the archetypical starburst galaxy
M82. Our results are based on a single spectrum at a position angle of 67.5
degrees through the K-band nucleus. We used the CO stellar absorption band head
at 2.29 {\mu}m (CO_2.29) to measure the rotation curve out to nearly 4 kpc
radius on both the eastern and western sides of the galaxy. Our data show that
the rotation curve is flat from 1 - 4 kpc. This stands in sharp contrast to
some previous studies, which have interpreted H I and CO emission-line
position-velocity diagrams as evidence for a declining rotation curve. The
kinematics of the Br\gamma, H_2, and He I emission lines are consistent with,
although characterized by slightly higher velocities than, the stellar
kinematics. We derived M82's mass distribution from our stellar kinematic
measurements and estimate its total dynamical mass is ~10^10 Msun. We measured
the equivalent width of CO_2.29 (W_2.29) as a function of distance from the
center of the galaxy to investigate the spatial extent of the red supergiant
(RSG) population. The variation in W_2.29 with radius clearly shows that RSGs
dominate the light inside 500 pc radius. M82's superwind is likely launched
from this region, where we estimate the enclosed mass is <= 2 x 10^9 Msun.
