We report the results of a radial velocity survey of planetary nebulae (PNe) located in the tidal features of the well-known interacting system NGC 5194/95 (M51). We find clear kinematic evidence that M51's north-western tidal debris consists of two discrete structures that overlap in projection -- NGC 5195's own tidal tail and diffuse material stripped from NGC 5194. We compare these kinematic data to a new numerical simulation of the M51 system and show that the data are consistent with the classic "single passage" model for the encounter, with a parabolic satellite trajectory and a 2:1 mass ratio. We also comment on the spectra of two unusual objects: a high-velocity PN that may be associated with M51's halo and a possible interloping high-redshift galaxy.