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(Abridged) We present an analysis of ionization and metal enrichment in theMagellanic Stream (MS), the nearest gaseous tidal stream, using HST/STIS andFUSE ultraviolet spectroscopy of two background AGN, NGC 7469 and Mrk 335. ForNGC 7469, we include optical spectroscopy from VLT/UVES. In both sightlines theMS is detected in low-ion and high-ion absorption. Toward NGC 7469, we measurea MS oxygen abundance [O/H]_MS=[OI/HI]=-1.00+/-0.05(stat)+/-0.08(syst),supporting the view that the Stream originates in the SMC rather than the LMC.We use CLOUDY to model the low-ion phase of the Stream as a photoionized plasmausing the observed Si III/Si II and C III/C II ratios. Toward Mrk 335 thisyields an ionization parameter log U between -3.45 and -3.15 and a gas densitylog (n_H/cm^-3) between -2.51 and -2.21. Toward NGC 7469 we derive sub-solarabundance ratios for [Si/O], [Fe/O], and [Al/O], indicating the presence ofdust in the MS. The high-ion column densities are too large to be explained byphotoionization, but also cannot be explained by a single-temperaturecollisional-ionization model (equilibrium or non-equilibrium). This suggeststhe high-ion plasma is multi-phase. Summing over the low-ion and high-ionphases, we derive conservative lower limits on the ratio N(total H II)/N(H I)of >19 toward NGC 7469 and >330 toward Mrk 335, showing that along these twodirections the vast majority of the Stream has been ionized. The presence ofwarm-hot plasma together with the small-scale structure observed at 21 cmprovides evidence for an evaporative interaction with the hot Galactic corona.This scenario, predicted by hydrodynamical simulations, suggests that the fateof the MS will be to replenish the Galactic corona with new plasma, rather thanto bring neutral fuel to the disk.

Exploring the Origin and Fate of the Magellanic Stream with Ultraviolet and Optical Absorption