The potential dip observed behind the shock front excited in an ion beam-plasma system in a double plasma device is shown to be the nonlinear solitary slow wave of the system. The experimental nonlinear velocity agrees with the predictions of a fluid model with cold ions. Furthermore, if the ion temperature is taken into account, the acceleration experienced by the ions in the dip and the resulting modification of the ion distribution explain the probe signals and probe characteristics. The existence of an ion acoustic instability localized inside the solitary slow wave is reported; the instability occurs at beam velocities lower than for the usual beam plasma instability.