A mechanism of the thermal and photochemical bleaching of merocyanine to spiropyran is proposed on the basis of CASSCF/CASPT2 calculations on the 6-(2-propenyliden)cyclohexadienone model system. Our results suggest that this photochemical transformation takes place in two steps. First, the initially pumped (1)(pi-pi*) S-2 undergoes radiationless decay to 1(n-pi*) S-1 via an extended S-2/S-1 conical intersection seam that runs approximately parallel to the trans-to-cis isomerization coordinate, a few kilocalories per mole higher in energy. Thus, S-2 -> S-1 internal conversion is possible at all values of the S-2 trans-to-cis reaction coordinate. Second, on the S-1 potential energy surface, there is a barrierless ring closure reaction path from the S-1 cis minimum that leads to a peaked S-1/S-0 conical intersection where the deactivation to the ground state takes place. The inertia of the moving nuclei then drives the system toward the ground-state minimum of the 2H-chromene product. Thus, the extended seam topology of the S-2/S-1 conical intersection and the coordinate of the branching space of the S-1/S-0 conical intersection are essential to explain the efficiency and high speed of this reaction.