A FORTRAN IV Monte Carlo program to calculate the neutron detection efficiency of plastic scintillators of rectangular or cylindrical geometry as a function of neutron energy and light collection threshold has been written and tested. The existing cross-section data for neutron-carbon interactions have been adjusted to give a good fit to existing measurements of efficiency. This document contains a complete description of the physics input and method of approach, together with detailed instructions for use of the program. A listing of the source code, the data deck, and output of a sample calculation are appended.