The study examines and compares optical photometric and UV (IUE) spectroscopic data on the active RS CVn star HK Lac gathered in the years 1979, 1980, and 1987. The photometry is modeled by representations of 'starspots' and, in general, it is found that the UV emission lines are stronger at times of spot maximum. The variable interstellar absorption, due to the Doppler motion of the Mg II emission lines from the active stellar component as they move across the interstellar absorption lines, is quantitatively discussed. It is shown that, for HK Lac, the effect produces a double-peaked modulation in the MG II h+k flux with an amplitude of 4.5 percent, i.e., considerably smaller than the observed modulation of about 40 percent. It is concluded that the variation in the UV line fluxes is stellar rather than interstellar in origin. It is shown that a well-defined anticorrelation exists between the IUE emission-line fluxes and the V band, suggestive of an association in longitude between photospheric spots and chromospheric plages. Four IUE high-dispersion spectra of MG II emission features are also examined.