  We present photometric and spectroscopic observations and analysis of SN
2021bxu (ATLAS21dov), a low-luminosity, fast-evolving Type IIb supernova (SN).
SN 2021bxu is unique, showing a large initial decline in brightness followed by
a short plateau phase. With $M_r = -15.93 \pm 0.16\, \mathrm{mag}$ during the
plateau, it is at the lower end of the luminosity distribution of
stripped-envelope supernovae (SE-SNe) and shows a distinct $\sim$10 day plateau
not caused by H- or He-recombination. SN 2021bxu shows line velocities which
are at least $\sim1500\,\mathrm{km\,s^{-1}}$ slower than typical SE-SNe. It is
photometrically and spectroscopically similar to Type IIb SNe during the
photospheric phases of evolution, with similarities to Ca-rich IIb SNe. We find
that the bolometric light curve is best described by a composite model of shock
interaction between the ejecta and an envelope of extended material, combined
with a typical SN IIb powered by the radioactive decay of $^{56}$Ni. The
best-fit parameters for SN 2021bxu include a $^{56}$Ni mass of $M_{\mathrm{Ni}}
= 0.029^{+0.004}_{-0.005}\,\mathrm{M_{\odot}}$, an ejecta mass of
$M_{\mathrm{ej}} = 0.61^{+0.06}_{-0.05}\,\mathrm{M_{\odot}}$, and an ejecta
kinetic energy of $K_{\mathrm{ej}} = 8.8^{+1.1}_{-1.0} \times 10^{49}\,
\mathrm{erg}$. From the fits to the properties of the extended material of
Ca-rich IIb SNe we find a trend of decreasing envelope radius with increasing
envelope mass. SN 2021bxu has $M_{\mathrm{Ni}}$ on the low end compared to
SE-SNe and Ca-rich SNe in the literature, demonstrating that SN 2021bxu-like
events are rare explosions in extreme areas of parameter space. The progenitor
of SN 2021bxu is likely a low mass He star with an extended envelope.
