We illustrate the transformation of terahertz plasmonics within an array of rectangular sub-wavelength holes (RSHs) into coherent and enhanced terahertz emission via Smith-Purcell effect. The radiative plasmonic modes within each RSH of the array are successively excited by an free-electron beam, which then generate coherent radiation by constructive interference. Compared with the case without taking plasmonics into consideration, the radiation field intensity is enhanced by more than an order of magnitude, affording a promising way of developing high-power terahertz radiation. We perform detailed analysis of the plasmonic modes within the RSH by using the dielectric waveguide theory, and the results are verified by numerical simulations. The influences of the RSH parameters on the radiation properties are revealed and discussed.