The research contained within this thesis focuses on understanding and interrogating nanometrically confined ultrafast plasmon-phonon interactions. Nanometric plasmonic confinement is achieved by coupling the charge oscillations of a single gold nanoparticle to a gold substrate. This confines the plasmonic field to the single nanometre gap separating the two and is known as the NanoPar- ticle on Mirror (NPoM) structure. The first half of this thesis demonstrates the viability of using the coupling between confined plasmonic and acoustic modes as a sensitive nanomechanical probe. Initially this coupling allows us to discover the NPoM “bouncing mode” by performing ultrafast pump- probe spectroscopy on single constructs. Thorough finite element method simulations allow us to create a simple analytical model relat- ing the nanoparticle-substrate contact area to the bouncing mode period. This means that by measuring the bouncing mode period of a single NPoM structure we can calculate the size of the contact; a task impossible by any other means. The second half of this thesis is dedicated to furthering our under- standing of ultrafast molecular-phonon plasmon interactions. To fa- cilitate this we develop and utilise a fully automated time-resolved Coherent Anti-Stokes Raman Spectroscopy (tr-CARS) setup to mea- sure an acceleration of the vibrational decay of 2-mercaptopyridine within NPoM from 0 . 96 ps (determined from bulk Raman linewidth measurements) to far below 0 . 5 ps . To understand the origin of this acceleration we perform a series of power dependent Surface En- hanced Raman Spectroscopy (SERS) measurements on over 1000 NPoM constructs. We determine the acceleration to be most likely due to anharmonic phonon coupling driven by the high phonon pop- ulations induced by ultrafast pulses in plasmonic cavities. The power- series also reveals the presence of a previously unknown saturation effect due to intermolecular anharmonicity.