This paper presents imaging, photometric, and spectroscopic observations of NGC 3921 = Mrk 430 gathered over many years with five optical telescopes. This luminous galaxy (M_V_= -22.8 for H_0_ = 50) at cz_hel_=5926 +/- 15 km s^-1^ features a single nucleus, a main body with complex fine structure (ripples, loops, fan-shaped protrusions), and a pair of ~100 kpc long, crossed tidal tails indicative of two former disk galaxies of near-equal mass. These galaxies have essentially merged. The main body of the remnant shows a typical post-starburst spectrum dominated in the blue by A 3-5 V stars. The inferred burst age is 0.5-1 Gyr and the burst strength ~10% (by mass). Surrounding the nucleus is extremely centrally concentrated ionized gas that can be traced out to ~12" (7 kpc), emits ~> 1.5 x 10^41^ ergs s^-1^ in Hα, and shows signs of both rotational and chaotic motions. The bright semistellar nucleus appears strikingly off-centered relative to the main body, which itself features "sloshing" isophotes. That is, the centers of successive isophotes shift position by ~>2 kpc, causing the nucleus to appear eccentric by up to 23% relative to a nearly half-light isophote. The luminous matter has clearly not yet equilibrated, and this merger remnant is dynamically young. Nevertheless, the mean light distribution of the main body is already well described by an r^1/4^ law. This distribution plus the luminosity, UBV colors, color gradients, velocity dispersion, spectroscopic line strengths, and fine-structure index all agree with the notion that NGC 3921, which is a member of a small, tight group of four galaxies, is a 0.7+/-0.3 Gyr old protoelliptical (reckoned since close passage that started the merger). Both it and its kin NGC 7252 are nearby analogs of distant galaxies with "E+A"-type spectra in Butcher-Oemler clusters. A search for star clusters and associations in NGC 3921 reveals 19 candidate OB associations, but only five candidate young globular clusters with M_V_ = -12 to -14. Thus, NGC 3921 appears to have distinctly fewer and certainly less luminous young globular clusters than NGC 7252. This less extreme population of young globulars may reflect a paucity of gas in one of the two merging component disks of this suspected S0-Sc or Sa-Sc merger (Hibbard & van Gorkom, AJ, in press). Such gas paucity may explain the weaker starburst and may have supplied fewer giant molecular clouds for globular cluster formation. Hence, the Hubble types and gas contents of component galaxies appear to play an important role in determining the cluster populations in merger remnants.