  We show that the physical degrees of freedom of the critical open string with
N=2 superconformal symmetry on the worldsheet are described by a self-dual
Yang-Mills field on a hyperspace parametrised by the coordinates of the target
space R^{2,2} together with a commuting chiral spinor. A prepotential for the
self-dual connection in the hyperspace generates the infinite tower of physical
fields corresponding to the inequivalent pictures or spinor ghost vacua of this
string. An action is presented for this tower, which describes consistent
interactions amongst fields of arbitrarily high spin. An interesting truncation
to a theory of five fields is seen to have no graphs of two or more loops.
