  We argue that the effective field theory on D3-branes in a plane-wave
background with 3-form flux is a nonlocal deformation of Yang-Mills theory. In
the case of NSNS flux, it is a dipole field theory with lightlike dipole
vectors. For an RR 3-form flux the dipole theory is strongly coupled. We
propose a weakly coupled S-dual description for it. The S-dual description is
local at any finite order in string perturbation theory but becomes nonlocal
when all perturbation theory orders are summed together.
