Sunlight plays a critical role in the development of emerging sustainable energy conversion and storage technologies. Light-induced CO2 reduction by artificial photosynthesis represents one of the most crucial challenges to produce renewable fuels and chemicals that do not degrade the environment. Synergetic interface interactions between plasmonic metal nanoparticles and semiconductor have been shown to lead to improved activities in CO2 photoreduction over a wide range of the solar spectrum. However, the effect of plasmonic resonances on the photo-induced charge transfer processes and their catalytic behaviour is still under debate, mainly due to the different timescales at which such processes occur. Here, we use a unique combination of advanced in-situ and time-resolved spectroscopies combined with theory to unravel the overall mechanism of photocatalytic CO2 reduction by Ag/TiO2 catalyst. Our findings provide evidence for the key factors determining the enhancement of photoactivity under UV and visible irradiation, which have important implications for the design of photocatalysts for solar energy conversion.