  We present extensive observations of SN 2018zd covering the first
$\sim450$\,d after the explosion. This SN shows a possible shock-breakout
signal $\sim3.6$\,hr after the explosion in the unfiltered light curve, and
prominent flash-ionisation spectral features within the first week. The unusual
photospheric temperature rise (rapidly from $\sim 12,000$\,K to above
18,000\,K) within the earliest few days suggests that the ejecta were
continuously heated. Both the significant temperature rise and the flash
spectral features can be explained with the interaction of the SN ejecta with
the massive stellar wind ($0.18^{+0.05}_{-0.10}\, \rm M_{\odot}$), which
accounts for the luminous peak ($L_{\rm max} = [1.36\pm 0.63] \times 10^{43}\,
\rm erg\,s^{-1}$) of SN 2018zd. The luminous peak and low expansion velocity
($v \approx 3300$ km s$^{-1}$) make SN 2018zd to be like a member of the LLEV
(luminous SNe II with low expansion velocities) events originated due to
circumstellar interaction. The relatively fast post-peak decline allows a
classification of SN 2018zd as a transition event morphologically linking
SNe~IIP and SNe~IIL. In the radioactive-decay phase, SN 2018zd experienced a
significant flux drop and behaved more like a low-luminosity SN~IIP both
spectroscopically and photometrically. This contrast indicates that
circumstellar interaction plays a vital role in modifying the observed light
curves of SNe~II. Comparing nebular-phase spectra with model predictions
suggests that SN 2018zd arose from a star of $\sim 12\,\rm M_{\odot}$. Given
the relatively small amount of $^{56}$Ni ($0.013 - 0.035 \rm M_{\odot}$), the
massive stellar wind, and the faint X-ray radiation, the progenitor of SN
2018zd could be a massive asymptotic giant branch star which collapsed owing to
electron capture.
