We consider the steady developed flow of a liquid layer having just passed the trailing edge of horizontal flat plate in the asymptotic limit of large Reynolds number. The body/interface forces at play are gravity acting vertically and constant surface tension on both sides of the layer. Two-tiered (internal) viscous-inviscid interaction accounts rigorously for the interplay of these and the viscous forces in the long-wave limit. Here a suitably formed reciprocal Weber number (T ) and reciprocal Froude number (G) form the sole control parameters. The solutions of the nonlinear interaction problem disclose a surprising richness of phenomena, distinguished by the range of values of T : antisymmetric (0 ≤ T ≤ 1/2) and symmetric (T > 1) capillary wave trains represent a nonlinear extension of classical Squire modes, where the latter ones can be seen as planar Rayleigh–Plateau modes; if 1/2 < T < 1, either gross flow reversal far downstream or a blow-up at some finite distance from the edge terminates the interactive flow description. Both scenarios are found to be separated by an unstable flow represented by a curve in the (T, G)-plane. Amongst others, we present a weakly nonlinear WKBJ analysis capturing the waves and briefly address the types of choking for T being close to 1/2 or 1 as well as the surprising intricacies associated with the Euler stage regularising the blow-up. Finally, the analogies and differences to the impact of capillarity on the developed flow through the orifice of a circular pipe are highlighted. Parts of this work were carried out jointly with Samuel Harris, Shiza Naqvi, Michael Nguyen (all UCL) and Georgios Pasias (Cyprus University of Technology).