Studying the dynamics of the polymer-plasticizer system poly(methyl methacrylate) (PMMA)/tri-m-cresyl phosphate (TCP), we find pronounced dynamic heterogeneities for the additive TCP with one- and two-dimensional P-31 NMR spectroscopy, whereas the dispersion of the dynamics of the polymer PMMA, studied simultaneously with H-2 NMR, remains essentially unchanged upon addition of the plasticizer TCP, despite a large decrease in the correlation time. TCP molecules reorient isotropically even in a rigid polymer matrix at temperatures well below the glass transition of the plasticized PMMA. Taking advantage of the very large time window accessible with the stimulated echo technique for the P-31 nucleus, we show that the orientational correlation functions of TCP change from Kohlrausch decays for pure TCP to quasi-logarithmic decays in the mixture, which resemble those from recent simulations with a bead-and-spring model [Moreno, A. J.; Colmenero, J. J. Chem. Phys. 2006, 124, 184906]. Two-dimensional spectra for TCP show that the dynamic heterogeneities of the plasticizer are transient in nature.