Transmittance spectra of a twisted-nematic liquid-crystal defect layer infiltrated in a one-dimensional photonic crystal are investigated in this paper. With the electrically controllable defect layer, the defect modes can be tuned with external voltage. Three different polarization angles are applied, yielding the E-, O-, and M-mode conditions. The M-mode twisted-nematic alignment leads to a series of independent defect modes that can be assigned to E- and O-modes. As an analyzer is utilized to obtain a crossed-polarizer scheme, the interplay between the photonic crystal and the mixed-mode twisted nematic liquid crystal causes the transmission spectrum to be distinctive from usual. We propose that this combination can be used as a very-narrow-band monochromatic selector with both wavelength and intensity tunability.