We analyze and experimentally demonstrate the design of an ultrathin quarter-wave plate based on detuned plasmonic nanoantennas. We explore the theoretical possibilities offered by thin metasurfaces to manipulate the polarization state of impinging light, and propose an optimal design to realize quarter-wave metasurface plates, analyzing their frequency and angular response. One of the most interesting features of our design is the large degree of linear polarization output for circularly polarized input over broad bandwidths in the visible, despite the extreme thinness. The geometry is implemented with one-step planar lithography, providing opportunities for integration in optical systems for polarization control, detection and sensing at the nanoscale.