We report the analysis of an extensive set of high-resolution spectroscopic observations of the massive binary system HD 149104. W compare different techniques to measure the radial velocities of the heavily blended absorption lines and we derive a new orbital solution. The absorption lines display strong variability that slightly affects the orbital solution and the determination of the spectral types of the components of the binary. We find that the primary is probably of spectral type O7.5I(f), while the secondary is most likely an ON9.7I supergiant. The secondary seems to be the most evolved component of the system and its current evolutionary status could best be explained if the system has undergone a Roche lobe overflow episode during the past. The secondary could actually still tie rather close to filling its critical volume and this could lead to an enhanced mass loss of the secondary. The spectrum of HD 149404 displays many emission lines some of which show phase-locked line profile variations. Ln particular, the HQ line displays a double-peaked morphology at orbital phases near conjunction. We investigate the radial velocity behaviour of the emission lines and we find that some of them must be formed in an interaction region. We propose a simple model where some of the optical emission lines arise in a heavily bended shock region.