Results of a chemodynamical study of the W-R ring nebula NGC 6888 around the WN6 star WR 136 are presented; the observations combine high spatial and spectral resolution covering most of the optical range in different zones of the nebula. The spectra show an emission system with three different velocity components: -64 km/s, +78 km/s, and +18 km/s. The first two correspond to the blue and redshifted parts of the shell, and the third is associated with the ambient ionized gas. Diagnostic diagrams indicate that the observed spectra are typical of purely photoionized nebulae, without any significant contribution of shock excitation. Chemical abundances of O/H, N/H, and He/H are derived for each velocity component; it is shown that while the ambient gas has nearly solar abundances, the expanding shell presents a remarkable O/H deficiency and strong overabundances of N/H and He/H. It is concluded that this nebula is composed mostly of stellar ejecta, possibly produced by an event of very high mass loss or shell ejection at, or very near, the RSG stage of the progenitor of WR 136 with an initial mass in the range 25-40 solar masses.