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We present high-cadence, comprehensive data on the nearby($D\simeq33\,\rm{Mpc}$) Type II SN 2017ahn, discovered within $\sim$1 day ofexplosion, from the very early phases after explosion to the nebular phase. Theobservables of SN 2017ahn show a significant evolution over the$\simeq470\,\rm{d}$ of our follow-up campaign, first showing prominent, narrowBalmer lines and other high-ionization features purely in emission (i.e. flashspectroscopy features), which progressively fade and lead to a spectroscopicevolution similar to that of more canonical Type II supernovae. Over the sameperiod, the decline of the light curves in all bands is fast, resembling thephotometric evolution of linearly declining H-rich core-collapse supernovae.The modeling of the light curves and early flash spectra suggest a complexcircumstellar medium surrounding the progenitor star at the time of explosion,with a first dense shell produced during the very late stages of its evolutionbeing swept up by the rapidly expanding ejecta within the first $\sim6\,\rm{d}$of the supernova evolution, while signatures of interaction are observed alsoat later phases. Hydrodynamical models support the scenario in which linearlydeclining Type II supernovae are predicted to arise from massive yellowsuper/hyper giants depleted of most of their hydrogen layers.

The early discovery of SN 2017ahn: signatures of persistent interaction in a fast declining Type II supernova