I. INTRODUCTION Since the historical work of Arago [1] and Pasteur [2], chirality (the handedness of nature) has generally been associated with optical activity, that is the rotation of the plane of polarisation of light passing through a medium lacking mirror symmetry [3, 4]. Optical activity is nowadays a very powerful probes of structural chirality in varieties of system. However, two-dimensional chiral structures, such as planar molecules, were not expected to display any chiral characteristics since simply turning the object around leads to the opposite handedness (we remind that a planar structure is chiral if it can not be brought into congruence with its mirror image unless it is lifted from the plane). This fundamental notion was recently challenged in a pioneering study where it was shown that chirality has a distinct signature from optical activity when electromagnetic waves interact with a 2D chiral structure and that the handedness can be recognized [5]. While the experimental demonstration was achieved in the giga-Hertz (mm) range for extended 2D structures, the question remained whether this could be achieved in the optical range since the laws of optics are not simply scalable when downsizing to the nanometer level. Here we report genuine optical planar chirality for a single subwavelength hole surrounded by left and right handed Archimedian spirals milled in a metallic film. Key to this finding is the involvement of surface plasmons, lossy electromagnetic waves at the metal surfaces, and the associated planar spatial dispersion [6, 7]. Our results reveal how, in a stringent and unusual way, this optical phenomenon connects concepts of chirality, reciprocity and broken time symmetry. We remind that partly boosted by practical motivations, such as the quest of negative refractive lenses [8] or the possibility to obtain giant optical activity for applications in optoelectronics, there is currently a renewed interest [8–18] in the optical activity in artificial photonic media with planar chiral structures. It was shown for instance that planar gam- madionic structures, which have by definition no axis of reflection but a four-fold rotational invariance [9, 11], can generate optical activity with giant gyrotropic factors [12, 16–18]. Importantly, and in contrast to the usual three dimensional (3D) chiral medium (like quartz and its helicoidal structure [3, 19]), planar chiral structures change their observed handedness when the direction of light is reversed through the system [9, 20]. This challenged Lorentz principle of reciprocity [4] (which is known to hold for any linear non magneto-optical media) and stirred up considerable debate [9, 10, 12, 21] which came to the conclusion that optical 2