The membrane protein bacteriorhodopsin contains all-trans-retinal in a binding site lined by amino acid side groups and water molecules that guide the photodynamics of retinal. Upon absorption of light, retinal undergoes a subpicosecond all-trans-->13-cis phototransformation involving torsion around a double bond. The main reaction product triggers later events in the protein that induce pumping of a proton through bacteriorhodopsin. Quantum-chemical calculations suggest that three coupled electronic states, the ground state and two closely lying excited states, are involved in the motion along the torsional reaction coordinate phi. The evolution of the protein-retinal system on these three electronic surfaces has been modelled using the multiple spawning method for non-adiabatic dynamics. We find that, although most of the population transfer occurs on a timescale of 300 fs, some population transfer occurs on a longer timescale, occasionally extending well beyond 1 ps.