  We present the revised and high quality CCD time series observations in 2017
and 2018 of the W UMa-type contact binary BH~Cas. The combination of our new
determinations of minimum times and those collected from the literatures reveal
a long-term period increase with a rate of $\textit{dP/dt}\,=\,+3.27 \times
10^{-7} \textrm{d}\,\textrm{yr}^{-1}$, which can be accounted for by angular
momentum transfer as the less-massive component loses its mass to the more
massive one. One cyclical oscillation ($A'=0.00300$~days and $P'=20.09$~yrs)
superimposed on the long-term period increase tendency is attributed to one
additional object that orbits BH Cas. The low-resolution spectra of this source
are observed at phase $\sim$0.0 and $\sim$0.5 in 2018 and 2019, and the
spectral types are identified as K3V$\,\pm\,$1 and G8V$\,\pm\,$2. According to
the luminosity contributions and spectra at these phases, the spectral type of
the primary star is close to K3V$\,\pm\,$1. We infer a cool spot on the hotter
and less-massive component to fit the asymmetry of light curves. In addition,
changes in the location, temperature, and area of spots on the secondary star
in different observations may indicate that the magnetic field activity of the
secondary is more active than that of the primary. Such strong spot activities
and optical flare are both detected at the same time from a W UMa-type star for
the first time. With the \emph{XMM-Newton} observations taken in 2003 and 2012,
the X-ray light curve is obtained and the X-ray spectra can be well described
with a black-body or a bremsstrahlung model. The soft X-ray luminosity of BH
Cas is estimated as 8.8\,--\,9.7$\,\times\,10^{30}$\,erg\,s$^{-1}$.
