One of the most widely studied model systems: for excited-state proton transfer (ESPT) is the 2-(2'-hydroxyphenyl)benzothiazole (HBT) molecule. This compound undergoes ultrafast ESPT followed by internal conversion to return to the ground state. In the present work, we simulate the nonadiabatic photochemistry of HBT using ab initio multiple spawning (AIMS) nuclear dynamics and, a complete active space configuration interaction (CASCI), method in conjunction with wave function-in-DFT embedding to obtain ground-and excited-state potential surfaces on-the-fly. Our simulation predicts ultrafast ESPT with a time constant of 48-54 fs and an excited-state lifetime of 1.7-1.8 ps. Following proton transfer HBT becomes trapped in a metastable keto structure on the S-1 state. Eventually, the molecule begins to twist and proceeds toward a seam of intersection with the ground state where internal conversion is highly efficient.