CASSCF and CASPT2 studies on the reaction mechanism of the photochromic ring-opening process of a spiropyran (SP) (1′,3′,3′-trimethylspiro-[2H-1-benzopyran-2,2′-indoline], also known as BIPS) have been performed and possible excited-state C–O (and C–N) bond cleavage pathways and S1-to-S0 nonadiabatic transition channels have been explored. (1) The C–O bond dissociation in SP does not follow a conical-intersection mechanism that has been proposed in a model study with a simplified benzopyran. The CASSCF-optimized crossing points are actually avoided crossings with a large S1–S0 energy gap at the CASPT2 level; thus, they could not act as efficient S1-to-S0 funnels. (2) C–O bond cleavage paths on S1 leading to both the CCC (cis–cis–cis with respect to the configuration around α, β, γ) and TCC (trans–cis–cis) intermediates of merocyanine (MC) are barrierless, in line with the experimentally observed ultrafast formation of MC. (3) An unexpected low-energy hydrogen-out-of-plane (HOOP) valley on the (π→σ*) surface was located not far from the C–O bond cleavage path and was suggested to be an efficient S1-to-S0 nonadiabatic decay channel. Triggered by the active HOOP mode, the molecule can easily access the S1–HOOP valley and then make a transition to the S0 surface through the narrow S1–S0 gap that exists in an extended region. Nonadiabatic decay through a conical intersection on C–N dissociation path as well as the HOOP funnel is responsible for high internal conversion yields of SP. These findings shedding light on the complex mechanism of SP–MC interconversion provide fundamental information for design spiropyran-based photochromic devices.