  We present extensive ground-based spectroscopy and HST imaging of 3C79, an FR
II radio galaxy associated with a luminous extended emission-line region
(EELR). Surface brightness modeling of an emission-line-free HST R-band image
reveals that the host galaxy is a massive elliptical with a compact companion
0.8" away and 4 magnitudes fainter. The host galaxy spectrum is best described
by an intermediate-age (1.3 Gyr) stellar population (4% by mass), superimposed
on a 10 Gyr old population and a power law (\alpha_{\lambda} = -1.8); the
stellar populations are consistent with super-solar metallicities, with the
best fit given by the 2.5 Z_sun models. We derive a dynamical mass of 4E11
M_sun within the effective radius from the velocity dispersion. The EELR
spectra clearly indicate that the EELR is photoionized by the hidden central
engine. Photoionization modeling shows evidence that the gas metallicity in
both the EELR and the nuclear narrow-line region is mildly sub-solar (0.3 - 0.7
Z_sun) -- significantly lower than the super-solar metallicities deduced from
typical active galactic nuclei in the SDSS. The more luminous filaments in the
EELR exhibit a velocity field consistent with a common disk rotation. Fainter
clouds, however, show high approaching velocities that are uncoupled with this
apparent disk rotation. The striking similarities between this EELR and the
EELRs around steep-spectrum radio-loud quasars provide further evidence for the
orientation-dependent unification schemes. The metal-poor gas is almost
certainly not native to the massive host galaxy. We suggest that the close
companion galaxy could be the tidally stripped bulge of a late-type galaxy that
is merging with the host galaxy. The interstellar medium of such a galaxy is
probably the source for the low-metallicity gas in 3C79.
