  The quasar SDSS 0956+5128 exhibits three distinct velocity components with
large offsets in emission: the systemic velocity of [O II], [O III] and [Ne
III] narrow lines have redshift $z=0.7142$; broad Mg II line is shifted by
$-1200$ km s$^{-1}$ with respect to the narrow lines; broad H$\alpha$, H$\beta$
lines are at $-4100$ km s$^{-1}$. We present new Hubble Space Telescope spectra
of Ly$\alpha$ and C IV emission lines and high-resolution images of the quasar.
The offsets of these lines are consistent with the velocity component of the
Balmer emission, and the photometry in optical and near-infrared wavelengths
does not show any signs of recent mergers in the host galaxy or irregularities
in the location of the quasar. The data do not confirm predictions of the
previous most-likely hypotheses involving a special orientation and morphology
of the quasar disk, such as in the recoiling black hole scenario, neither it is
consistent with accretion disk winds. Instead, based on the cumulative
evidence, we propose a new scenario, in which the broad line region is in the
state of outflow caused by a strong shock wave, with a supernova as a possible
event for producing the shock ejecta.
