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We describe and analyze Hubble Space Telescope (HST) observations oftransient emission near 4680 {\AA} in Eta Car, reported earlier by Steiner &Damineli (2004). If, as seems probable, this is He II $\lambda$4687, then it isa unique clue to Eta Car's 5.5-year cycle. According to our analysis, severalaspects of this feature support a mass-ejection model of the observedspectroscopic events, and not an eclipse model. The He II emission appeared inearly 2003, grew to a brief maximum during the 2003.5 spectroscopic event, andthen abruptly disappeared. It did not appear in any other HST spectra before orafter the event. The peak brightness was larger than previously reported, andis difficult to explain even if one allows for an uncertainty factor of order3. The stellar wind must provide a temporary larger-than-normal energy supply,and we describe a special form of radiative amplification that may also beneeded. These characteristics are consistent with a class of mass-ejection orwind-disturbance scenarios, which have implications for the physical structureand stability of Eta Car.Comment: 47 pages (including all appendices, tabs, & figs), 9 figures, 3  tables; submitted to Astrophysical Journal (2005 March 29), accepted for  publication in Ap

On the HeiiEmission in η Carinae and the Origin of Its Spectroscopic Events