  We analyze the data obtained when the Konus-Wind gamma-ray spectrometer
detected a giant flare in SGR 1806-20 on December 27, 2004. The flare is
similar in appearance to the two known flares in SGR 0526-66 and SGR 1900+14
while exceeding them significantly in intensity. The enormous X-ray and
gamma-ray flux in the narrow initial pulse of the flare leads to almost
instantaneous deep saturation of the gamma-ray detectors, ruling out the
possibility of directly measuring the intensity, time profile, and energy
spectrum of the initial pulse. In this situation, the detection of an
attenuated signal of Compton back-scattering of the initial pulse emission by
the Moon with the Helicon gamma-ray spectrometer onboard the Coronas-F
satellite was an extremely favorable circumstance. Analysis of this signal has
yielded the most reliable temporal, energy, and spectral characteristics of the
pulse. The temporal and spectral characteristics of the pulsating flare tail
have been determined from Konus-Wind data. Its soft spectra have been found to
contain also a hard power-law component extending to 10 MeV. A weak afterglow
of SGR 1806-20 decaying over several hours is traceable up to 1 MeV. We also
consider the overall picture of activity of SGR 1806-20 in the emission of
recurrent bursts before and after the giant flare.
