  We present optical imaging and spectroscopy of supernova (SN) LSQ13fn, a type
II supernova with several hitherto-unseen properties. Although it initially
showed strong symmetric spectral emission features attributable to
\ion{He}{ii}, \ion{N}{iii}, and \ion{C}{iii}, reminiscent of some interacting
SNe, it transitioned into an object that would fall more naturally under a type
II-Plateau (IIP) classification. However, its spectral evolution revealed
several unusual properties: metal lines appeared later than expected, were
weak, and some species were conspicuous by their absence. Furthermore, the line
velocities were found to be lower than expected given the plateau brightness,
breaking the SNe~IIP standardised candle method for distance estimates. We
found that, in combination with a short phase of early-time
ejecta-circumstellar material interaction, metal-poor ejecta, and a large
progenitor radius could reasonably account for the observed behaviour.
Comparisons with synthetic model spectra of SNe~IIP of a given progenitor mass
would imply a progenitor star metallicity as low as 0.1\,Z$_{\odot}$. LSQ13fn
highlights the diversity of SNe~II and the many competing physical effects that
come into play towards the final stages of massive star evolution immediately
preceding core-collapse.
