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(Abridged) We present a new high-resolution (7 km/s FWHM) echelle spectrum of3C 351 obtained with STIS. 3C 351 lies behind the low-latitude edge ofhigh-velocity cloud Complex C, and the new spectrum provides accuratemeasurements of O I, Si II, Al II, Fe II, and Si III absorption lines at thevelocity of the HVC. We use collisional and photoionization models to deriveionization corrections; in both models we find that the overall metallicity Z =0.1 - 0.3 Z_{solar} in Complex C, but nitrogen must be underabundant. The ironabundance indicates that Complex C contains very little dust. The absorbing gasprobably is not gravitationally confined. The gas could be pressure-confined byan external medium, but alternatively we may be viewing the leading edge of theHVC, which is ablating and dissipating as it plunges into the Milky Way. O VIcolumn densities observed with FUSE toward nine QSOs/AGNs behind Complex Csupport this conclusion: N(O VI) is highest near 3C 351, and the O VI/H I ratioincreases substantially with decreasing latitude, suggesting that thelower-latitude portion of the cloud is interacting more vigorously with theGalaxy. The other sight lines through Complex C show some dispersion inmetallicity, but with the current uncertainties, the measurements areconsistent with a constant metallicity throughout the HVC. However, all of theComplex C sight lines require significant nitrogen underabundances. Finally, wecompare the 3C 351 sight line to the sight line to the nearby QSO H1821+643 tosearch for evidence of outflowing Galactic fountain gas that could be mixingwith Complex C. We find that the intermediate-velocity gas detected toward 3C351 and H1821+643 has a higher metallicity and may well be a fountain/chimneyoutflow from the Perseus spiral arm.

Complex C: A Low-Metallicity, High-Velocity Cloud Plunging into the Milky Way