  We present optical photometric and low-resolution spectroscopic observations
of the Type II plateau supernova (SN) 2008in, which occurred in the outskirts
of the nearly face-on spiral galaxy M 61. Photometric data in the X-rays,
ultraviolet and near-infrared bands have been used to characterize this event.
The SN field was imaged with the ROTSE-IIIb optical telescope about seven days
before the explosion. This allowed us to constrain the epoch of the shock
breakout to JD = 2454825.6. The duration of the plateau phase, as derived from
the photometric monitoring, was ~ 98 days. The spectra of SN 2008in show a
striking resemblance to those of the archetypal low-luminosity IIP SNe 1997D
and 1999br. A comparison of ejecta kinematics of SN 2008in with the
hydrodynamical simulations of Type IIP SNe by Dessart et al. (2010) indicates
that it is a less energetic event (~ 5$\times10^{50}$ erg). However, the light
curve indicates that the production of radioactive $^{56}$Ni is significantly
higher than that in the low-luminosity SNe. Adopting an interstellar absorption
along the SN direction of $A_V$ ~ 0.3 mag and a distance of 13.2 Mpc, we
estimated a synthesized Ni mass of ~ 0.015 $M_{\odot}$. Employing
semi-analytical formulae (Litvinova & Nadezhin 1985), we derived a pre-SN
radius of ~ 126$R_{\odot}$, an explosion energy of ~ 5.4$\times10^{50}$ erg and
a total ejected mass of ~ 16.7$M_{\odot}$. The latter indicates that the zero
age main-sequence mass of the progenitor did not exceed 20$M_{\odot}$.
Considering the above properties of SN 2008in, and its occurrence in a region
of sub-solar metallicity ([O/H] ~ 8.44 dex), it is unlikely that fall-back of
the ejecta onto a newly formed black hole occurred in SN 2008in. We therefore
favor a low-energy explosion scenario of a relatively compact, moderate-mass
progenitor star that generates a neutron star.
