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We study the sudden optical and ultraviolet (UV) brightening of 1ES 1927+654,which until now was known as a narrow-line active galactic nucleus (AGN). 1ES1927+654 was part of the small and peculiar class of "true Type-2" AGN, whichlack broad emission lines and line-of-sight obscuration. Our high-cadencespectroscopic monitoring captures the appearance of a blue, featurelesscontinuum, followed several weeks later by the appearance of broad Balmeremission lines. This timescale is generally consistent with the expected lighttravel time between the central engine and the broad-line emission region in(persistent) broad-line AGN. Hubble Space Telescope spectroscopy reveals noevidence for broad UV emission lines (e.g., CIV1549, CIII]1909, MgII2798),probably owing to dust in the broad-line emission region. To the best of ourknowledge, this is the first case where the lag between the change in continuumand in broad-line emission of a "changing-look" AGN has been temporallyresolved. The nature and timescales of the photometric and spectral evolutiondisfavor both a change in line-of-sight obscuration and a change of the overallrate of gas inflow as driving the drastic spectral transformations seen in thisAGN. Although the peak luminosity and timescales are consistent with those oftidal disruption events seen in inactive galaxies, the spectral properties arenot. The X-ray emission displays a markedly different behavior, with frequentflares on timescales of hours to days, and will be presented in a companionpublication.

1ES 1927+654: An AGN Caught Changing Look on a Timescale of Months