The fundamental understanding of the subtle interactions between molecules and plasmons is of great significance for the development of plasmon-enhanced spectroscopy(PES) techniques with ultrahigh sensitivity.However, this information has been elusive due to the complex mechanisms and difficulty in reliably constructing and precisely controlling interactions in well-defined plasmonic systems. Herein, the interactions in plasmonic nanocavities of filmcoupled metallic nanocubes (NCs) are investigated. Through engineering the spacer layer,m olecule-plasmon interactions were precisely controlled and resolved within 2nm. Efficient energy exchange interactions between the NCs and the surface within the 1-2 nm range are demonstrated. Additionally, optical dressed molecular excited states with ah uge Lamb shift of % 7meV at the single-molecule (SM) level were observed. This work provides ab asis for understanding the underlying molecule-plasmon interaction, paving the wayf or fully manipulating light-matter interactions at the nanoscale.