  V838 Mon has undergone one of the most mysterious stellar outbursts on
record, with (a) a large amplitude (Delta B ~ 10 mag) and multi-maxima
photometric pattern, (b) a cool spectral type at maximum becoming cooler and
cooler with time during the descent, until it reached the never-seen-before
realm of L-type supergiants, never passing through optically thin or nebular
stages, (c) the development of a spectacular, monotonically expanding
light-echo in the circumstellar material, and (d) the identification of a
massive and young B3V companion, unaffected by the outburst. In this talk we
review the photometric and spectroscopic evolution during the first three full
years of outburst, the light-echo development and infer the nature of the
progenitor, which was brighter and hotter in quiescence than the B3V companion
and with an inferred ZAMS mass of about 65 Msun.
