We report a rigorous, time-dependent study of the three-dimensional HCN → HNC isomerization. Wavepackets are prepared by applying an impulse to three bend-excited stationary states that are localized in the HCN well. The time dependence of the wavepackets is obtained by expansion of the initial wavepacket in a basis of 900 vibrational eigenstates and subsequent propagation in this basis. The vibrational eigenstates were calculated previously using an accurate ab initio potential [Bowman, J. M.; Gazdy, B.; Bentley, J. A.; Lee, T. J.; Dateo, C. E. J. Chem. Phys. 1993, 97, 308]. The time-dependent density to be in the HNC well is calculated for many initial impulses, and time-averaged reaction probabilities are calculated as functions of both the average energy of the full wavepacket and the average energy of the components of the wavepacket that isomerize. The contribution of tunneling to the isomerization is calculated and shown to be mainly due to delocalized states with energies below the ground-state adiabatic barrier to isomerization. The cumulative reaction probability is also calculated and shown to have the expected RRKM steplike structure.