[en] The authors present a scalar-field quantum theory of multiwave mixing in a medium consisting of stationary two-level atoms. The theory is based on a set of quantum c-number Langevin equations generalized to a nondegenerate multimode field. A method is developed to solve for the c-number atomic polarization to any order in the number of modes with the assumption that the amplitude of each mode evolves slowly compared with the characteristic decay time of the atomic polarization. From this atomic polarization, the authors get the c-number Langevin equations of motion for the field annihilation operators. The solutions for the field annihilation operators do not directly give the spatial propagation of waves. An approximate quantum treatment of the spatial propagation of the multimode field is also developed