Using bidifferential calculus, we derive a vectorial binary Darboux transformation for the first member
of the ``negative'' part of the AKNS hierarchy. A reduction leads to the first ``negative flow" of the
NLS hierarchy, which in turn is a reduction of a rather simple nonlinear complex PDE in two dimensions, with a leading
mixed third derivative. This PDE may be regarded as describing geometric dynamics of a complex scalar field
in one dimension, since it is invariant under coordinate transformations in one of the two independent variables.
We exploit the correspondingly reduced vectorial binary Darboux transformation to generate multi-soliton
solutions of the PDE, also with additional rational dependence on the independent variables, and
on a plane wave background. This includes rogue waves.