  Outflowing winds of multiphase plasma have been proposed to regulate the
buildup of galaxies, but key aspects of these outflows have not been probed
with observations. Using ultraviolet absorption spectroscopy, we show that
"warm-hot" plasma at 10^{5.5} K contains 10-150 times more mass than the cold
gas in a poststarburst galaxy wind. This wind extends to distances >68
kiloparsecs, and at least some portion of it will escape. Moreover, the
kinematical correlation of the cold and warm-hot phases indicates that the
warm-hot plasma is related to the interaction of the cold matter with a hotter
(unseen) phase at >>10^{6} K. Such multiphase winds can remove substantial
masses and alter the evolution of poststarburst galaxies.
