Singlet fission (SF) could dramatically increase the efficiency of organic solar cells by producing two triplet excitons from each absorbed photon. While this process has been known for decades, most descriptions have assumed the necessity of a charge-transfer intermediate. This ab initio study characterizes the low-lying excited states in acene molecular crystals in order to describe how SF occurs in a realistic crystal environment. Intermolecular interactions are shown to localize the initially delocalized bright state onto a pair of monomers. From this localized state, nonadiabatic coupling mediated by intermolecular motion between the optically allowed exciton and a dark multi-exciton state facilitates SF without the need for a nearby low-lying charge-transfer intermediate. An estimate of the crossing rate shows that this direct quantum mechanical process occurs in well under 1 Ps in pentacene. In tetracene, the dark multi-exciton state is uphill from the lowest singlet excited state, resulting in a dynamic interplay between SF and triplet-triplet annihilation.