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