On the basis of an extensive ab initio electronic structure study of the ground and excited-state potential energy surfaces of the naphthalene radical cation (N.+), we propose a mechanism for its ultrafast nonradiative relaxation from the second excited state (D-2) down to the ground state (D-0), which could explain the experimentally observed photostability [Zhao, L.; Lian, R.; Shkrob I. A.; Crowell, R. A.; Pommeret, S.; Chronister, E. L.; Liu, A. D.; Trifunac, A. D. J. Phys. Chem. A., 2004, 108, 25]. The proposed photophysical relaxation pathway involves internal conversion from the D-2 state down to the D-0 state via two consecutive, accessible, sloped conical intersections (CIs). The two crossings, D-0/D-1 and D-1/D-2, are characterized at the complete active space self-consistent field (CASSCF) level. At this level of theory, the D-0/D-1 crossing is energetically readily accessible, while the D-1/D-2 CI appears too high in energy to be involved in internal conversion. However, the inclusion of dynamic correlation effects, via single point CASPT2 calculations including excitations out of the valence pi- and sigma-orbitals, lowers the D-0 and D-2 state energies with respect to D-1. Extrapolations at the CASPT2 level predict that the D-1/D-2 crossing is then significantly lower in energy than with CASSCF indicating that with a higher-level treatment of dynamic correlation it may be energetically accessible following vertical excitation to D-2. N.+ is proposed as one of the species contributing to a series of diffuse infrared absorption bands originating from interstellar clouds. Understanding the mechanism for photostability in the gas phase, therefore, has important consequences for astrophysics.