  We present observations of SN 2020fqv, a Virgo-cluster Type II core-collapse
supernova (CCSN) with a high temporal resolution light curve from the
Transiting Exoplanet Survey Satellite (TESS) covering the time of explosion;
ultraviolet (UV) spectroscopy from the Hubble Space Telescope (HST) starting
3.3 days post-explosion; ground-based spectroscopic observations starting
1.1~days post-explosion; along with extensive photometric observations. Massive
stars have complicated mass-loss histories leading up to their death as CCSNe,
creating circumstellar medium (CSM) with which the SNe interact. Observations
during the first few days post-explosion can provide important information
about the mass-loss rate during the late stages of stellar evolution. Model
fits to the quasi-bolometric light curve of SN 2020fqv reveal ~0.23 $M_{\odot}$
of CSM confined within ~1450 $R_{\odot}$ ($10^{14}$ cm) from its progenitor
star. Early spectra (<4 days post-explosion), both from HST and ground-based
observatories, show emission features from high-ionization metal species from
the outer, optically thin part of this CSM. We find that the CSM is consistent
with an eruption caused by the injection of $\sim$$5\times 10^{46}$ erg into
the stellar envelope $\sim$300 days pre-explosion, potentially from a nuclear
burning instability at the onset of oxygen burning. Light-curve fitting,
nebular spectroscopy, and pre-explosion \textit{HST} imaging consistently point
to a red supergiant (RSG) progenitor with $M_{\rm ZAMS}$$\approx$$13.5$--$15 \,
M_{\odot}$, typical for SN~II progenitor stars. This finding demonstrates that
a typical RSG, like the progenitor of SN 2020fqv, has a complicated mass-loss
history immediately before core collapse.
