Most molecules and dielectric materials have characteristic bond vibrations or phonon modes in the mid-infrared regime. However, infrared absorption spectroscopy lacks the sensitivity for detecting trace analytes due to the low quantum efficiency of infrared sensors. Here, we report mid-infrared photothermal plasmonic scattering (MIP-PS) spectroscopy to push the infrared detection limit toward nearly a hundred molecules in a plasmonic nanocavity. The plasmon scattering from a nanoparticle-on-film cavity has extremely high sensitivity to the spacing defined by the analyte molecules inside the nanogap. Meanwhile, a 1000-fold infrared light intensity enhancement at the bond vibration frequency further boosts the interaction between mid-IR photons and analyte molecules. MIP-PS spectroscopic detection of nitrile or nitro group in ~130 molecules was demonstrated. This method heralds potential in ultrasensitive bond-selective biosensing and bioimaging.